For evolution, what is important is that you survive and have good health long enough to have offsprings. The vast majority of people, even in modern times, have their children before 50 years old (because women have this hard limit in their fertility). So there was never an evolution pressure to make our body function propertly after that time.
Children who don't have extended family, such as grandparents, to care for them are more likely to die, either of of predators (if parents are out hunting) or of hunger (if parents stick around to protect their children).
