I cant speak for other clans, but I'm a part of a big gaming community and we do require 18+, not only for maturity (yes, there are some very mature teens, and there are also some very unmature people over 18) but it's also for legal reason. That is also the reason many sites asks if you are over 13, since if you are under that age you are not allowed to post personal information about yourself (or something like that). With a +18 age limit the guild does not have to worry about legal implications dealing with minors.