The answer for me to both is yes. I have some experience.
I did not say they can never get back to where they were or exceed. You are changing the narrative.
Your initial premise was quite simple, you thought it sounded fishy that someone back from a mission would not be advantaged over a 19 yr old.
It is a fact that you will be worse off, given a high level athlete.
It's nonsensical to argue otherwise.
A more apropos comparison would be to say who would be in better shape a 21 yr old rm or that same individual who has trained for those two years.
It is evident.