“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.”

— From the “pop up” version of the Tumblr TOS. Interestingly, it’s not included in the “official” version here. Sneaky sneaky!