I must respectfuly disagree.
The Evolution argument is valid for old people too. If having more NAD+ would be such an advantage, evolution would have already invented a way to get it... right?
The gene spreading argument doesn't convince me as men can spread their gene pretty much until the day they die.
I am not sure about this. Its probably true on an individual level: The longer you live, the more you can spread your genes.
But how is it on the level of a group of people? Lets say you have a tribe A in the stone age, all of which get mature with 15 and keep having offsprings until they are very very old. Another tribe B also gets mature with 15, but people die with 30. Which one will be more sucessful on the long run?
Tribe A will have a slower rate of evolution because the age difference between generations is higher. It might even have problems with incest, when 70-year old are sexually as active as 15-year olds. Younger individuals will also compete more for food with the older ones. Tribe B on the other hand will always have a younger population, fast evolution and less genetic errors.
Besides, evolution has already made it so that women can only have children while they are relatively young. From an evolutionary view, whats the point of keeping their health high for a long time after they raised their children? Whats the point of keeping the health of older men high, when there are plenty of younger men available, ready to fertilize? For society, there is no point in that, they just compete for ressources.
Ok, this was a pure evolutionary standpoint, its of course not what we want. My only point is, that its pretty clear that its an evolutionary advantage when young individuals are healthy, therefore evolution will probably optimize it. Its not so clear for much older people.