To at least try to protect the children from predators on the web.
Another social site that little girls are getting on by going around the 13-year rule has seen posts by them including their actual street address (yes, full address! Not just number and street but city, state) and email address as well as phone numbers!
The pedophiles are experts at getting information so they can trap a little girl or boy and have their way with them.
Federal laws are stricter, regarding the information that websites can collect from children under the age of 13. In most cases it is easier (and less costly) for a site to simply add an age restriction, rather than try to comply with the increased regulations.
They are too young. It is to prevent little children from getting lured into by older men who search the web to find and talk to children, mainly female. Also they may be to young to understand Facebook. I think it is for their own good.