Age for Fasting in the Catholic Church: According to Church law, all persons 14 years and older are bound to abstain from meat on the days prescribed (Ash Wednesday, Good Friday, and Fridays throughout the year unless they are solemnities, or the conference of bishops has established another penance). Children under 18 are not bound to fast; adults under 60 years of age are bound to fast on the days prescribed (the only specific days prescribed for the whole Church are Ash Wednesday and Good Friday, though fasting more often is encouraged). Of course, those really unable to fast for reasons of health are excused from fasting.
From Summa Theologiae II-II 147 and In IV Sent. d. 15, q. 3, a. 1, qa. 2
1. It seems that fasting is not a virtue, since every, both good and bad persons, are fasting before they eat. But virtue is done only by good persons. Therefore fasting is not a virtue.
2. Further, fasting means not only abstaining from superfluity of food, but also from what is necessary. But he who withholds from himself necessary food, gives himself an occasion of death; for there is no difference, as Jerome says, whether you kill yourself in a long or a short time. Therefore since no one is permitted to kill himself, it seems to no way is permitted to fast, and so fasting is not virtuous.
I respond, an action is virtuous due to its being directed by reason to a noble good. And this is true of fasting. For we fast for three purposes: (1) to restrain the desires of the flesh; (2) to raise the mind to contemplate sublime things; (3) to make satisfaction for our sins. These are good and noble things, and so fasting is virtuous.
To the first objection it should be said that the “fast of nature”, which everyone is said to be doing before they eat, consists simply in a negation (i.e., that someone hasn't eaten yet, and isn't eating now). So this fasting cannot be called a virtuous act, but only that fasting by which a person somehow abstains from food for a reasonable purpose.
To the second objection it should be said that food can be necessary in two ways. First, for preserving life. And food necessary in this way one may not abstain from by fasting, just as one may not kill oneself. But this necessary is very little, since nature is content with small amounts. In another way food is necessary for preserving the strength of the body. And this may be understood in two ways: (1) in one way, as sufficient strength with respect to the things that one must do because of one's position, or because of the company of those with whom one lives; and we may also not abstain from food necessary in this way, since this would be to offer a fast of theft (from those to whom one is obliged), if someone on account of fasting were hindered from doing to the works to which he would otherwise be obliged. Hence Jerome says, “he who immoderately afflicts his body either by eating too little food, or by taking too little sleep, offers a sacrifice of theft.” Also if a man's abstains so much from food that he is hindered from more useful deeds, even if he is not bound to do them, his fasting is injudicious, even if not forbidden. Hence Jerome says, “rational man loses dignity, if he prefers fasting to charity, or vigils to integrity of the senses.” (2) Strength of body may be taken in the sense of the body's being disposed as well as possible; and since the flesh in its vigor is only with difficulty subjected to the spirit, it may be praiseworthy to withhold from the body what is necessary for strength in this sense, even if one could licitly take it. And withholding this nourishment does not significantly (multum) hasten death, since the human body more frequently becomes deathly ill due to superfluity than due to a deficiency. Hence Galen says that abstinence is the supreme medicine. It is also a matter of experience that those who abstain from food frequently live longer; hence this abstaining from food cannot be called an occasion of death, since it may do either: i.e., prolong or shorten life. But abstaining from what is unnecessary in either of these senses mentioned above, is obligatory by the virtue of temperance.
From the Introduction to the Devout Life III, ch. 23
If you are able to fast, you will do well to observe some days beyond what are ordered by the Church, for besides the ordinary effect of fasting in raising the mind, subduing the flesh, confirming goodness, and obtaining a heavenly reward, it is also a great matter to be able to control greediness, and to keep the sensual appetites and the whole body subject to the law of the Spirit; and although we may be able to do but little, the enemy nevertheless stands more in awe of those whom he knows can fast. The early Christians selected Wednesday, Friday and Saturday as days of abstinence. Do you follow therein according as your own devotion and your director’s discretion may appoint.
... I disapprove of long and immoderate fasting, especially for the young. I have learned by experience that when the colt grows weary it turns aside, and so when young people become delicate by excessive fasting, they readily take to self-indulgence.... A want of moderation in the use of fasting, discipline and austerity has made many a one useless in works of charity during the best years of his life...
Fasting and labor both exhaust and subdue the body. If your work is necessary or profitable to God’s Glory, I would rather see you bear the exhaustion of work than of fasting. Such is the mind of the Church, who dispenses those who are called to work for God or their neighbor even from her prescribed fasts. One man finds it hard to fast, another finds it as hard to attend the sick, to visit prisons, to hear confessions, preach, minister to the afflicted, pray, and the like. And the last hardship is better than the other; for while it subdues the flesh equally, it brings forth better fruit. And as a general rule it is better to preserve more bodily strength than is absolutely necessary, than to damage it more than is necessary. Bodily strength can always be lowered if needful, but we cannot restore it at will. It seems to me that we ought to have 219 in great reverence that which our Saviour and Redeemer Jesus Christ said to His disciples, “Eat such things as are set before you.” 119 To my mind there is more virtue in eating whatever is offered you just as it comes, whether you like it or not, than in always choosing what is worst; for although the latter course may seem more ascetic, the former involves greater submission of will, because by it you give up not merely your taste, but your choice; and it is no slight austerity to hold up one’s likings in one’s hand, and subject them to all manner of accidents.
From the Treatise on the Love of God XI, ch. 13
I may fast in Lent, either from charity in order to please God; or from obedience, because it is a precept of the Church; or from sobriety; or from diligence, in order to study better; or from prudence, to make some saving which is required; or from chastity, in order to tame the flesh; or from religion, the better to pray. Now, if I please, I may make a collection of all these intentions, and fast for them all together: but in that case there must be good management to place these motives in proper order. For if I fasted chiefly in order to save money, rather than from obedience to the Church; if to study well rather than to please God;—who does not see that I pervert right and order, preferring my own interest before obedience to the Church and the pleasure of my God? To fast in order to save is good, to fast in order to obey the Church is better, to fast in order to please God is best: but though it may seem that with three goods one cannot make a bad; yet he who should place them out of order, preferring the less to the better, would without doubt commit an irregularity deserving of blame.
From his Message for Lent 2009
We might ask ourselves what value and
meaning there is for us Christians in depriving ourselves of
something that in itself is good and useful for our bodily
sustenance. The Sacred Scriptures and the entire Christian tradition
teach that fasting is a great help to avoid sin and all that leads to
it. For this reason, the history of salvation is replete with
occasions that invite fasting.... Since all of us are weighed down by
sin and its consequences, fasting is proposed to us as an instrument
to restore friendship with God.
Jesus brings to light the profound
motive for fasting, condemning the attitude of the Pharisees, who
scrupulously observed the prescriptions of the law, but whose hearts
were far from God. True fasting, as the divine Master repeats
elsewhere, is rather to do the will of the Heavenly Father, who “sees
in secret, and will reward you” (Mt 6,18). ... The true
fast is thus directed to eating the “true food,” which is
to do the Father’s will (cf. Jn 4,34).
Fasting represents an important
ascetical practice, a spiritual arm to do battle against every
possible disordered attachment to ourselves. Freely chosen detachment
from the pleasure of food and other material goods helps the disciple
of Christ to control the appetites of nature, weakened by original
sin, whose negative effects impact the entire human person.
It is good to see how the ultimate goal of fasting is to help each one of us to make a complete gift of self to God. May every family and Christian community use well this time of Lent, therefore, in order to cast aside all that distracts the spirit and grow in whatever nourishes the soul, moving it to love of God and neighbor. I am thinking especially of a greater commitment to prayer, lectio divina, recourse to the Sacrament of Reconciliation and active participation in the Eucharist, especially the Holy Sunday Mass.
Matthew 6:16-18 "And when you fast, do not look dismal, like the hypocrites, for they disfigure their faces that their fasting may be seen by men. Truly, I say to you, they have received their reward. But when you fast, anoint your head and wash your face, that your fasting may not be seen by men but by your Father who is in secret; and your Father who sees in secret will reward you.
Matthew 9:14-15 Then the disciples of John came to him, saying, "Why do we and the Pharisees fast, but your disciples do not fast?" 15 And Jesus said to them, "Can the wedding guests mourn as long as the bridegroom is with them? The days will come, when the bridegroom is taken away from them, and then they will fast.
Tobit 12:8 Prayer is good when accompanied by fasting, almsgiving, and righteousness.
Isaiah 58:3-8 `Why have we fasted, and thou seest it not? Why have we humbled ourselves, and thou takest no knowledge of it?' Behold, in the day of your fast you seek your own pleasure, and oppress all your workers. Behold, you fast only to quarrel and to fight and to hit with wicked fist. Fasting like yours this day will not make your voice to be heard on high. Is such the fast that I choose, a day for a man to humble himself? Is it to bow down his head like a rush, and to spread sackcloth and ashes under him? Will you call this a fast, and a day acceptable to the LORD? "Is not this the fast that I choose: to loose the bonds of wickedness, to undo the thongs of the yoke, to let the oppressed go free, and to break every yoke? Is it not to share your bread with the hungry, and bring the homeless poor into your house; when you see the naked, to cover him, and not to hide yourself from your own flesh? Then shall your light break forth like the dawn, and your healing shall spring up speedily; your righteousness shall go before you, the glory of the LORD shall be your rear guard.
Brief notes on Aquinas's teaching on fasting
Provided here for those interested in pursuing at greater length St. Thomas Aquinas's teaching on fasting
1. Against the concupiscence of the flesh: almsgiving against the concupiscence of the eyes, and prayer against the pride of life. (IV Sent 15:1:4:3; ST I-II 108:3 ad 4; ST II-II 147:1)
Fasting is a virtue that restrains desire; for sin occurs more readily by excess in delights, and so it is virtuous to restrain the bodily appetites. (IV Sent 15:3:1:1)
It is for this restraint that we abstain from meat. (IV Sent 15:3:4) And also that we eat once a day (ST II-II 147:6)
We abstain from meat and animal products because, being most suited to our bodies, they are most delightful, and because they incline us most to lust. (ST II-II 147:8) (Meat is generally more delightful than fish, and therefore meat is forbidden.)
2. To raise to mind to contemplate divine things (ST II-II 147:1) Hence we fast before Easter. (ST II-II 147:5)
3. Thus we are prepared for grace (IV Sent 15:3:3:1 corpus et ad 6); hence we fast before Easter.
4. Satisfaction for sins. Almsgiving is more satisfactory than prayer, and prayer than fasting. (IV Sent 15:2:2B). But fasting and prayer also guard from sin by force of inclination (Ibid ad 1) (Four periods of fasting on the rogation days are to satisfy for sins; IV Sent 15:3:3:1 ad 5). It is for this satisfaction that we eat only once per day (IV Sent 15:3:4:2)
1. Fasting must not take away so much food as is necessary for the preservation of life.
2. Nor as much as is necessary for sufficient strength to perform those things that one’s state requires, or that are required for living with others. (ST II-II 147:1 ad 2)
3. Nor as much as is necessary for better works (though this is not sinful, it is indiscreet).
4. But fasting can praiseworthily take away that amount which makes the body strongest, since the body in its full strength is with difficulty subject to the spirit.
5. One can fast on any day; but in it is unfitting to fast on the Lord’s day, inasmuch as the whole Church rejoices, and one should not show signs of sorrow. (IV Sent 15:3:3:2)
1. Fasting is a certain affliction of the body (IV Sent 15:1:4:3 ad 5); every other affliction may be considered as falling under fasting.
2. Fasting belongs to the natural law inasmuch as one is bound to abstain from food to the extent necessary to keep the flesh from rebelling against the spirit. Not absolutely, but inasmuch as men tend to need fasting for this purpose. (IV Sent 15:3:1D; ST II-II 147:3)
3. There is a fasting of the humility of tribulation (fast of affliction), which does not belong to perfect men; this fast is prescribed by the Church. In contrast, the fast of exultation, made in the joy of the mind caught up to spiritual things, which belongs to perfect men, is not prescribed by the Church. (ST II-II 147:4 ad 5, 147:5 ad 3), (With the possible exception of Pentecost; Cf. IV Sent 3:3:1 ad 5)