[Kirk Y]

Have a look at this, if you need something more precise:

function bfrMin($foo, $bar)
{
    $foo = $foo == '' ? '00-00-0000' : $foo;
    $bday = explode('-', $foo);    
    $year = vbdate('Y', TIMENOW, false, false);
    $month = vbdate('n', TIMENOW, false, false);
    $day = vbdate('j', TIMENOW, false, false);
    if ($year > $bday[2] AND $bday[2] != '0000')
    {
        $age = $year - $bday[2];
        if ($month < $bday[0] OR ($month == $bday[0] AND $day < $bday[1]))
        {
            $age--;
        }
    }else{
        return 0;
    }
    return $age >= $bar ? 2 : 1;
}

$foo must contain the user's birthday in 00-00-0000 format.
$bar must contain a minimum age.

So, this: bfrMin('01-01-1989', 15)

Would return 0 if the user hasn't supplied a birthday to vBulletin, 1 if the user is younger than 15 years, or 2 if the user is 15 years old or older.