Objection 1. It would seem that all will not rise again of the same, namely the youthful age. Because God will take nothing pertaining to man's perfection from those who rise again, especially from the blessed. Now age pertains to the perfection of man, since old age is the age that demands reverence. Therefore the old will not rise again of a youthful age.
Objection 2. Further, age is reckoned according to the length of past time. Now it is impossible for past time not to have passed. Therefore it is impossible for those who were of greater age to be brought back to a youthful age.
Objection 3. Further, that which belonged most to the truth of human nature in each individual will especially rise again in him. Now the sooner a thing was in man the more would it seem to have belonged to the truth of human nature, because in the end, through the strength of the species being weakened the human body is likened to watery wine according to the Philosopher (De Gener. i). Therefore if all are to rise again of the same age, it is more fitting that they should rise again in the age of childhood.
On the contrary, It is written (Ephesians 4:13): "Until we all meet... unto a perfect man, unto the measure of the age of the fulness of Christ."
Now Christ rose again of youthful age, which begins about the age of thirty years, as Augustine says (De Civ. Dei xxii). Therefore others also will rise again of a youthful age.
Further, man will rise again at the most perfect stage of nature. Now human nature is at the most perfect stage in the age of youth. Therefore all will rise again of that age.
I answer that, Man will rise again without any defect of human nature, because as God founded human nature without a defect, even so will He restore it without defect. Now human nature has a twofold defect. First, because it has not yet attained to its ultimate perfection. Secondly, because it has already gone back from its ultimate perfection. The first defect is found in children, the second in the aged: and consequently in each of these human nature will be brought by the resurrection to the state of its ultimate perfection which is in the youthful age, at which the movement of growth terminates, and from which the movement of decrease begins.
Reply to Objection 1. Old age calls for reverence, not on account of the state of the body which is at fault; but on account of the soul's wisdom which is taken for granted on account of its being advanced in years. Wherefore in the elect there will remain the reverence due to old age on account of the fulness of Divine wisdom which will be in them, but the defect of old age will not be in them.
Reply to Objection 2. We speak of age not as regards the number of years, but as regards the state which the human body acquires from years. Hence Adam is said to have been formed in the youthful age on account of the particular condition of body which he had at the first day of his formation. Thus the argument is not to the point.
Reply to Objection 3. The strength of the species is said to be more perfect in a child than in a young man, as regards the ability to transform nourishment in a certain way, even as it is more perfect in the seed than in the mature man. In youth, however, it is more perfect as regards the term of completion. Wherefore that which belonged principally to the truth of human nature will be brought to that perfection which it has in the age of youth, and not to that perfection which it has in the age of a child, wherein the humors have not yet reached their ultimate disposition.
Source: Summa Theologica (New Advent)