The restriction started on March 3rd, so it's fairly recent.
Super Mario Boards
Goombungee
THE END
...If you read the Terms of Service before registering like you should, you'll see that way before that it already stated 'This Service is only available to users who are at least 13 years old. If you are younger than this, please do not register for this Service.'
xoalora
but if you are 12 years old
THE END
if you are 12 years old you can't use the forums