Tumblr, in their recently-updated Terms of Service that all logged in users are being asked to agree to.
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.
I’d add, if you’re less than 13, try going outside. But this is good.