No individual under the age of thirteen (13) may use the Services, provide any personal information to Tumblr, or otherwise submit personal information through the Services (including, for example, a name, address, telephone number, or email address). You may only use the Services if you can form a binding contract with Tumblr and are not legally prohibited from using the Services.
A binding contract? A lot to expect from a 13-year-old.
In slightly smaller print, on a slightly darker background:
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. “But I’m, like, 12.9 years old!” you plead. Nope, sorry. If you’re younger than 13, don’t use Tumblr. Ask your parents for a Playstation 4, or try books.