This happens among all mammals! Humans just are not very good at it, we are tolerant of the wrong things. Among other mammals an extremely anti-social or irrationally aggressive individual are shunned, even killed, because their inability to be a team player makes them a danger. This, to keep the breeding population strong and healthy. It is not necessarily about age, either. Among animals, the group will protect the very young and accomodate the slower moving elders. In a wolf pack for example. the entire pack travels in a way that cushions the elders or infirm. But a psycho that causes chaos and inappropriate aggression or endangers the pack? Dispatched.

This happens among all mammals! Humans just are not very good at it, we are tolerant of the wrong things. Among other mammals an extremely anti-social or irrationally aggressive or weak and sickly individual are shunned. This, to keep the breeding population strong and healthy.