This is a common term for U.S. contracts due to the Children's Online Privacy Protection Rule ("COPPA"). Basically, there are legal restrictions on collecting data from children under 13 years of age so in order to keep things simple, companies prefer not to collect ANY information from them at all.

Now... normally Mozilla doesn't collect personal information, but if you create a Firefox account or sign up for the newsletter or register for this site, Mozilla requests your email address. This is considered personal information, so I guess children under 13 should NOT use these services. Kind of late to learn that now (if you are under 13).