By definition, COPPA is for age 13. I suppose it would be possible to change the age limit, though.
I believe the way to change it, would be to open ubb_registration.cgi, search for:
$COPPADate = "$GotTime{mday} $months[$GotTime{JSMonth}] " . ($GotTime{JSYear}-13);
and change it to:
$COPPADate = "$GotTime{mday} $months[$GotTime{JSMonth}] " . ($GotTime{JSYear}-18);
That's just from looking at the file and guessing, though... you'd have to test it to make sure. Or wait till someone more familiar with the registration routines can answer this.
---Skorpion