I know how you feel. I lied about my age when I was too young (entering 18+ groups at age 13), but I feel extra uncomfortable now in certain fandoms where I feel too old. It's mostly just bandom. I'm not only older than most of the fans (I've been at concerts where the fans on either side of me were literally my age combined), I'm also older than half of the people in the bands. It's awkward.

But it definitely varies. I feel age appropriate in most of my fandoms.