So I've seen this on peachyhxlland so all credits go to her and you should definitely check her works out!

Ever since I got into Five fanfics and that some people do a 58 year old x 30's something and yes they're technically adults but Five could literally get a senior discount if he wanted to.

So instead of spending 45 years in the apocalypse I was thinking either 35 years or 25 years give or take.

He would still be older than his siblings by 10 years or 8 years. ( still haven't decided on which one )

And when Season 2 old Five comes along just imagine him a bit younger than 58

As for their physical appearance since Natalia would be placed in a coma at 17 trapped in her 17 year old body, I'll be doing the same with Five.  He will still go to the future at 13,

And since Aidan Gallagher, the actor portraying Five was 14 when he started filming and yes he's 20, still makes me and an also the actor uncomfortable to be writing smuts for a minor at the time. ( I can't even write it to begin with, I thought about for my other works ) So instead they will just have a couple of make out sessions.