Also not sure if possible but user is 17yo and comes of age of 18 to able to automatically switch user to adult? hence why I'd like to use date/age field if possible or another dob type field. If not possible at least I know when user becomes of age I can manually change usergroup.

Now I've tried doing this but with date field but options such as date range - only past date and offsets doesn't seem to make any effect on the calendar (does this feature work yet?) because it doesn't stop user from saying he is born in 2017.. I might be doing it wrong thou but did notice its the same on your registration on this site.

triggerFieldDate_of_birthUpdate($user,&$data,$changed,$isNew){

    $timestamp = strtotime('-18 years');

    $testDate = date('Y-m-d', $timestamp);

    if($data['date_of_birth']>$testDate){

        $usergroup=<insert id of teen group>

        JsnHelper::addUserToGroup($user,$usergroup);

        $usergroup=<insert id of adult group>

        JsnHelper::removeUserFromGroup($user,$usergroup);

    }

    else{

        $usergroup=<insert id of adult group>

        JsnHelper::addUserToGroup($user,$usergroup);

        $usergroup=<insert id of teen group>

        JsnHelper::removeUserFromGroup($user,$usergroup);

    }

}

Have you tried to set in date field this parameter:
- Date Range: Only Past date
- Offset Year: -14

Just tried it again and yeah still not working its like the parameters are totally ignored...