If you intentionally left your prayers or fasts, then, your oldest son is not responsible to make qaza.
He is only responsible, if you missed prayers due to ignorance, such as you did not pray while you were sick, thinking that you have an excuse, or if your prayers were void, because your wudou was void…etc.
Some people do not perform wudou correctly, and they realize this after years, then they start to perform qaza, but they die before completing it.