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?
The only reason I remember that I’m 64 is that I will be 65 on my next birthday. That’s kind of a milestone for a number of reasons and it’s easy to remember. The same was true when I was about to turn 50. Other than that, anything after 30 is kind of blurry