The key issue in your statement is about dying YOUNG.
If someone dies AFTER they have had all their children, then their death will not have much of an impact on the overpopulation issue.
Maybe you should consider the concept in this way... since everyone dies, the real factor in long-term population GROWTH is how many births there are per person before they die.
If on average every person alive has 1 child (each couple has 2 children), before dying, then the population will remain reasonably static, regardless of age of death.
However, if on average every person alive has 1.2 children, before dying, then the population will grow tremendously over time, regardless of age of death.
It is quite related to the concept of compound interest. Killing off the old people who are not going to have any more children, while indeed a large number, is not as significant as the long term effect of continuing growth.
So, before you berate me about needing to have a brain cell, perhaps you should figure out what the fuck you are talking about. Maybe if you shelved the hatred you wouldn't be making a fool of yourself so often?
On average, people who smoke, drink and so on, can have a ton of children before they eventually die at an age at which they usually don't have any more children anyway.