Because your son filed I-485 sucessfully at age 19. His age is frozen at 19 for the immigration purpose.
Had he missed the Sep 2005 filing of I-485, his age would continue to be counted until the next time visa number available, which is spring of 2007, in such case, he would problably had aged out, even after substracting I-140 processing time.
Had he missed the Sep 2005 filing of I-485, his age would continue to be counted until the next time visa number available, which is spring of 2007, in such case, he would problably had aged out, even after substracting I-140 processing time.