@HolyPhoenix Here's more detail, from my admittedly limited experience. I'd be interested to see the younger ones you've run into.
Discourse: lead developer is 40-something, team average is 30-40. User base is the same average, although some younger.
Q2A: old lead developer is probably 40 or so, new developer is the same. User base is highly international with some language barrier because of that.
Vanilla: user base averages 30-35
Askbot: developer is 30s or 40s, not many frequent posters in community forum.
OSQA: not many regular users or development, so likely older
Moot.it: not sure, but could be younger. However not forum with similar functions as the rest
phpBB, vb, xenforo, etc: pretty large developer bases and user bases, spread between wide age ranges, averages 25-40 maybe? Lots of old school forum users though.