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?
If you are using a piece of knowledge rarely, it is less likely to remember it.
It doesn’t matter whether it’s some word from a foreign language you are trying to learn, a math formula or names and birthdays, even your own, or whatever.
I’m also terrible with names. But then again, I rarely use them. I even tend to forget my own birthday and would miss it if there weren’t people around me reminding me of it.
If other people know their age and birthday immediatly, that’s probably because they are regularly thinking about it. For names it is therefore helpful to use the name of persons you just got acquainted with very often in conversations with them.