Objection 1. It would seem that all are bound to keep the fasts of the Church. For the commandments of the Church are binding even as the commandments of God, according to Luke 10:16, "He that heareth you heareth Me." Now all are bound to keep the commandments of God. Therefore in like manner all are bound to keep the fasts appointed by the Church.
Objection 2. Further, children especially are seemingly not exempt from fasting, on account of their age: for it is written (Joel 2:15): "Sanctify a fast," and further on (Joel 2:16): "Gather together the little ones, and them that suck the breasts." Much more therefore are all others bound to keen the fasts.
Objection 3. Further, spiritual things should be preferred to temporal, and necessary things to those that are not necessary. Now bodily works are directed to temporal gain; and pilgrimages, though directed to spiritual things, are not a matter of necessity. Therefore, since fasting is directed to a spiritual gain, and is made a necessary thing by the commandment of the Church, it seems that the fasts of the Church ought not to be omitted on account of a pilgrimage, or bodily works.
Objection 4. Further, it is better to do a thing willingly than through necessity, as stated in 2 Corinthians 9:7. Now the poor are wont to fast through necessity, owing to lack of food. Much more therefore ought they to fast willingly.
On the contrary, It seems that no righteous man is bound to fast. For the commandments of the Church are not binding in opposition to Christ's teaching. But our Lord said (Luke 5:34) that "the children of the bridegroom cannot fast whilst the bridegroom is with them [Vulgate: 'Can you make the children of the bridegroom fast, whilst the bridegroom is with them?']." Now He is with all the righteous by dwelling in them in a special manner [Cf. I, 8, 3], wherefore our Lord said (Matthew 28:20): "Behold I am with you... even to the consummation of the world." Therefore the righteous are not bound by the commandment of the Church to fast.
I answer that, As stated above (I-II, 90, 2; I-II, 98, 2,6), general precepts are framed according to the requirements of the many. Wherefore in making such precepts the lawgiver considers what happens generally and for the most part, and he does not intend the precept to be binding on a person in whom for some special reason there is something incompatible with observance of the precept. Yet discretion must be brought to bear on the point. For if the reason be evident, it is lawful for a man to use his own judgment in omitting to fulfil the precept, especially if custom be in his favor, or if it be difficult for him to have recourse to superior authority. on the other hand, if the reason be doubtful, one should have recourse to the superior who has power to grant a dispensation in such cases. And this must be done in the fasts appointed by the Church, to which all are bound in general, unless there be some special obstacle to this observance.
Reply to Objection 1. The commandments of God are precepts of the natural law, which are, of themselves, necessary for salvation. But the commandments of the Church are about matters which are necessary for salvation, not of themselves, but only through the ordinance of the Church. Hence there may be certain obstacles on account of which certain persons are not bound to keep the fasts in question.
Reply to Objection 2. In children there is a most evident reason for not fasting, both on account of their natural weakness, owing to which they need to take food frequently, and not much at a time, and because they need much nourishment owing to the demands of growth, which results from the residuum of nourishment. Wherefore as long as the stage of growth lasts, which as a rule lasts until they have completed the third period of seven years, they are not bound to keep the Church fasts: and yet it is fitting that even during that time they should exercise themselves in fasting, more or less, in accordance with their age. Nevertheless when some great calamity threatens, even children are commanded to fast, in sign of more severe penance, according to Jonah 3:7, "Let neither men nor beasts... taste anything... nor drink water."
Reply to Objection 3. Apparently a distinction should be made with regard to pilgrims and working people. For if the pilgrimage or laborious work can be conveniently deferred or lessened without detriment to the bodily health and such external conditions as are necessary for the upkeep of bodily or spiritual life, there is no reason for omitting the fasts of the Church. But if one be under the necessity of starting on the pilgrimage at once, and of making long stages, or of doing much work, either for one's bodily livelihood, or for some need of the spiritual life, and it be impossible at the same time to keep the fasts of the Church, one is not bound to fast: because in ordering fasts the Church would not seem to have intended to prevent other pious and more necessary undertakings. Nevertheless, in such cases one ought seemingly, to seek the superior's dispensation; except perhaps when the above course is recognized by custom, since when superiors are silent they would seem to consent.
Reply to Objection 4. Those poor who can provide themselves with sufficient for one meal are not excused, on account of poverty, from keeping the fasts of the Church. On the other hand, those would seem to be exempt who beg their food piecemeal, since they are unable at any one time to have a sufficiency of food.
Reply to Objection 5. This saying of our Lord may be expounded in three ways. First, according to Chrysostom (Hom. xxx in Matth.), who says that "the disciples, who are called children of the bridegroom, were as yet of a weakly disposition, wherefore they are compared to an old garment." Hence while Christ was with them in body they were to be fostered with kindness rather than drilled with the harshness of fasting. According to this interpretation, it is fitting that dispensations should be granted to the imperfect and to beginners, rather than to the elders and the perfect, according to a gloss on Psalm 130:2, "As a child that is weaned is towards his mother." Secondly, we may say with Jerome [Bede, Comment. in Luc. v] that our Lord is speaking here of the fasts of the observances of the Old Law. Wherefore our Lord means to say that the apostles were not to be held back by the old observances, since they were to be filled with the newness of grace. Thirdly, according to Augustine (De Consensu Evang. ii, 27), who states that fasting is of two kinds. one pertains to those who are humbled by disquietude, and this is not befitting perfect men, for they are called "children of the bridegroom"; hence when we read in Luke: "The children of the bridegroom cannot fast [Hom. xiii, in Matth.]," we read in Matthew 9:15: "The children of the bridegroom cannot mourn [Vulgate: 'Can the children of the bridegroom mourn?']." The other pertains to the mind that rejoices in adhering to spiritual things: and this fasting is befitting the perfect.
Source: Summa Theologica (New Advent)