Pokemon? I remember watching the original series when it first aired... pretty sure 1997 or 1998. I even remember buying the first season on VHS for my younger brother...
Did not overly care for it myself... never really got into the games, and I think I only ever saw the first three movies.
I know he was 10 originally... but I agree, wasn't there some league he had to go to and be 12 to get into it?
Hardly the only anime to have such a thing... Ranma for example, in the anime despite multiple Christmas parties and seasonal changes Kuno in the last season is still proclaiming himself the same age and grade as in the first season.