[Nyororin]

A bit late to the topic but...

I think people grow out of acting like overenthusiastic idiots about their interests or hobbies. No matter what the interest or hobby is, I think the majority of people you are going to encounter who act visibly like major fans, talk constantly about said interest, etc etc... Are going to be young, or at the very least quite immature.
As people get older the "world" sort of expands - additional interests and the maturity to match behavior to the audience develops. Not to mention life itself changes - as an independent adult it would be a bit hard to "live for anime" as it seems so many young fans do.