I'm in the US, so I can't say with complete certainty, but I've heard some chatter about it having to do with the laws where the website and these forums are hosted. Because of the integration of the site and minecraft and that you must create an acount on the site to play on the server, the laws in the country it's hosted (France?) say you must be 13 to join the forums. That, I believe, is why.
If I'm wrong or missing details though, please correct me.