Cause children can be annoying, more often than teenagers, albeit anyone can be annoying.
I've met kids half our age that are more mature than most 30 year olds I know. I don't believe you need to be a certain age to join a clan.
I mean if I was running a clan again, I'd probably set an age limit. Not because I think every kid is immature, but just because the vast majority will be and it'd be too much work to sift through so many applications to find the few that would be capable.