Please Help: COPPA Registration

[dotagious]

I would like to use the COPPA system, but ONE thing. Right now it auto calculates the on or before birth date during registration. The date figures out if someone is 13 years old or younger. I would like to use the same system, but change the age it tried to calculate to 18. So if someone is 17 years old or younger, they would be placed in the COPPA usergroup. Anyone know how and where I would change this?

[dotagious]

Hrm.... after playing with the COPPA feature, I don't think this is going to work. So, I'll put it to you like this:

I want to seperate all members 17 years of age and under, and those 18 years of age and older by usergroup. I want to do this because I would like to restrict access to some forums to those 17 and under. How could I do this so each member is placed in the correct usergroup (out of the two usergroups) during after filling out the appropriate info during registration?