This hack requires you to edit the register.php in your forum root directory.
open register.php
Find
Code:
if ($year < ($current['year'] - 13) OR ($year == ($current['year'] - 13) AND $month <= $current['month']))
Replace with
Code:
if ($year < ($current['year'] - 18) OR ($year == ($current['year'] - 18) AND $month <= $current['month']))
then that should do it
you may also want to change some of the coppa variables if you got admincp=>languages & phrases=>search in phrases.
type coppa in the search box then in the search in... section click Phrase Text and Phrase Variable Name
and this will bring up the coppa variables
hope this is of some help