A1 has a genetic trait that makes old age a terribly painful experience, and the moment of death a horrifying ordeal. Far worse than what most experience.
A2 becomes scared of death. As A2 gets older, they behave chaotically because of the trauma from watching A1 die painfully and the fear that they’ll experience the same fate.
A2 does not take good care of A3.
I hope this example (unscientifically) shows that there are likely some strong trends toward genetic traits that support a peaceful end of life period.
I suspect there are also multigenerational trends toward older members dying before they become a drain on a tribe’s resources. An obvious historically-useful benefit, that now leaves us worried sick about our aging parents (that they’ll slip and fall, and activate the “end of life” phase, where they’d have passed away if it weren’t for medical intervention).