Yeah, it tends to be, though there's often considered to be an overlap because older people will adjust to the more "accepted" social mores faster. I had two older sisters who whipped me into line if I did anything stupid luckily so I never really got a bad reputation for being pre- or young-teen in such a structured part of fandom, and from memory I would have been by far the youngest for quite some time - I remember the first time I posted smut I got some people going "omg but you're a baby!" (Actually I'm not sure it was even very poorly written, it was just sort of stereotypical sex by numbers - blow job to orgasm, male-on-female oral to orgasm while the guy recovers, then PIV sex. Oh, young me.) It was a Star Trek group, and the local fen were mostly into Highlander, which are both really old fandoms, too.