Boards that have 13 as the minimum age are more likely to be all or mostly 30+ if they have been around long enough, after a certain point. They almost never get new (younger) members.
Proof:
NeoGAF
Resetera
GameFAQs
IGN

Boards that advertise as "adult" are more likely to continually solicit new younger members, who want to make their identity an adult one.