As far as i know. Ghouls were "born" when the bombs fell. So.. in fallout 3 they are over 200 years old. They just age much much slower than normal people. It really depends how much a ghoul does age compared to normal people in fallout lore.
Like 30 ghoul years=1 human year ?
yeah, it was always my understanding that most, if not all, ghouls became ghouls by the initial bombs dropping, as indicated by the burnt, disfigured skin and face. and any children exposed to this would have been like 200 years old by FO3, and even in FO1 they are still fairly old.
oh, and the reason for no children ghouls being born is that all the radiation has turned the ghouls infertile, which i think is also stated, and if not is actually true.