(Aquinas starts by listing a number objections, which he then responds to. Here’s the first part of the response.)
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.