We haven't implemented COPPA support (yet?), but tied with the DOB requirement option is a configurable age requirement. It'd be trivial to force members to be 18+.
I think the initial COPPA rules were not to just block users under 13, but if they were 13 you would need to get parental permission to allow them in, so you would have to ask for their parents email address and then would email them for approval...
But I think a simple block for under 13's is good enough, unless there are sites out there that actually target the younger audience and still need to abide by COPPA...