A bit of humor in the Tumblr Terms of Service:
You have to be at least 13 years old to use Tumblr. We’re serious: it’s a hard rule, based on U.S. federal and state legislation, even if you’re 12.9 years old. If you’re younger than 13, don’t use Tumblr. Ask your parents for an Xbox or try books.
As legal documents go, this one is particularly clear and well written.
Ed Bott (@ wordpress.com) 
I love it when folks do like these guys and get really down to Earth on their terms. Makes life so much easier for everyone.