Why can't kids under 13 ask questions?

[brian 2010]

Because of legal and safety reasons, the Community Guidelines dictates that anyone who wants to participate on Answers must be at least 13 years old. Anyone younger than that will risk getting reported and subsequently get their accounts suspended. It's been done for their own good because pedophiles are lurking among the members of this community and would otherwise take advantage of them.

Edit: It's also found in the Terms of Service, Part 3, under the Heading "Your Registration Obligations."