I don't know if this is much use to you.
Age Lock Hack
Before Global Family Forum became Ask Family I had this issue of having 2 completely different areas - under 17's and Adults and the way I did it on GFF was to create a different usergroup for the under 17's which they were put in when they registered. I had COPPA set up so they had to give date of birth (only covers for under 13's).
At the end of the day no matter what measures you put into place it all depends on the prospective members or a member being honest with you.
Tinks