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?
In your younger years, you have a lot more to look forward to that’s tied to your age: first decade, teen years, driving, getting a job, voting, making your own decisions, second decade, and drinking age. After that, the only things that are still age locked are at 25 when you can rent a car and your insurance goes down, the other decades, and legal age requirements for reaching certain political offices. There just isn’t much else to look forward to at a specific age, so it’s not as important for your brain to remember.
Misremembering people’s names is not related to the age. A chunk of the population is really great at remembering names and another chunk remembers faces better. I’m more of a face person, I will most likely recognize you, but might not remember your name.