It's's not explicit but it's at the tutorial part of the game (A.k.a the part almost nobody really reads anymore), if you do the math, mentioned in the years Vic has before he goes ded. Balitz kinda explains it in a nutshell version of what I would say. Assuming it's a normal human speed pregnancy, and you age like a human would since you're ofc always part human (Or full). But yeah, you're def 19 years and a few months old in the game. But unlike some I don't really have issue with it.
Can assure most NPCs will be older than the PC. Few exceptions being Embry from what I'm reading here, and maybe Geoff, for he looks like a college-age looking kind of guy. Not counting Aurora obviously because I'm talking sexable characters of legality.
As for the age of literally everyone else outside those mentioned in the past along with here, I dunno. I don't really think about that stuff in porn games, or when I'm playing this game in general, I go by looks unless specified in some way in the text itself. I mean heck I could assume from Ciarans art of him that he looked like a middle-aged 40-50 year DILF, idr his actual age but he def looks it.
Savin said before that Astra's 18 iirc, and I think Shade says herself Astra's around the PC's age and that she turned 18 recently ingame.
Considering Savin plans to probably have Astra sexable at some point (TBD), it's better to stay at the guess of that she's legal age when we meet her. Probably just acts a tad naive despite being an adult now, but that's fine.