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’ve been getting it wrong since I was around 23. It’s often a case of “hang on, am I 23 or 24?” where I’m off by one year. It was easier during university because I knew of my peers’ ages and that made my age mean something relative to them. Now, age doesn’t come up regularly at all, so I’m more likely to forget and have to work it out as you describe