Why is facebooks age limit 13?

Answer 1

The Children's On-line Privacy Protection Act (COPPA, sections 1301 and following of Title XIII of the US Code) stringently restricts the kind of information which can be collected, and, more importantly, disseminated, about children under the age of thirteen. In particular, any collection of personally identifiable information (such, as, say, a status message) requires "verifiable parental consent", as defined in the act.

As a result, most internet services like Facebook, simply don't permit children under the age of thirteen to participate.