i think its because its a teen site and ur a preteen if ur 12... but if u ask me , personally i think its beacause they want ppl who would contribute.. aka "ppl who would bring drama with them beacuse fanpop thrives on drama.. so.. a younger person probbly didnt experince enough in life to have drama.. soo yeah :\
I think that maybe some (and please note that I'm saying some) people who are younger than 13 may not seem mature enough to contribute in some of the conversations. I mean, I see some things postato da younger users and I think "You really have no idea do you". Having saying that, I'm 13, and half my anno are 12 anno olds, and I suppose that a lot of them don't act any younger than me.
I see your point, and I do think that there are some 16-year-olds who are less mature than 12 anno olds, but I guess that 13 is the guideline, and I think that it's a fair one too. Though I think that if te act just as mature on fanpop as most, then there's no problem, and no one has anything to complain about, but that's just my opinion.