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?
I wouldn’t say I frequently forget my age, but it happens. It’s only worrying if you can’t calculate it on demand. For what it’s worth, after your late 20’s or so you’re basically the same person mentally and what changes are details like how much a day spent working in an office chair screws up your neck and back.