They will generally be reset to be some years away from retirement, depending on how old they were before and far into your playthrough you are. Lemme know if it feels bad when you're playing.
Completely honest: Totally random. The age does reset even between chapters. For example i could hire one of my legacy chars in chapter 1 with age 21 and in chapter 2 it went up to 40. That is also the point where i think it is not worth anymore to hire them if i get them for only twü chapters.