I'm still in my 20s, but as of a few years ago I started forgetting what's my exact age. I always have to stop and recalculate it each time someone asks me. I get asked fairly infrequently, but when I do it's a bit weird/embarrassing that I have to say "wait, let me calculate". (I know when I was born, of course.)
It seems as if there's no good reason I'd remember it, since it changes all the time and it is rarely mentioned in practice. But others, including people much older than myself, know their own age immediately.
I'm also terrible at remembering people's names, I don't know if that could be related?
It's probably normal to forget. I am probably the odd one in that I'm always very aware of my age and I'm almost 40. I'm also acutely aware of the ages of people around me, and (very mildly) uncomfortable if I don't know someone's age.
TL:DR Maladaptive behavior that is however ultimately harmless since I don't bother people.
I was a "gifted child"; was always like 2 school years ahead, started uni at 15 and every single person I met would praise me for being the youngest. I was immature so it got to my head. I also have always looked much younger than I am, which also invites comments. Finally, I also have AuDHD and I'm constantly anxious about not acting my age and being too immature. So I try to look at other people's ages to guide me in how I'm supposed to behave.
All of this is maladaptive and I've gotten a lot better with time, and I'm still working on it, but I'm not particularly bothered about the actual fact of having an interest in people's ages. I make sure I don't ask them about it or bother them about it, but many just offer this information on their own.