Humans have selective adaptations to reduce resource competition between older and younger members of populations - examples are menopause and testosterone levels.

Part of the reason it benefited us that some but not all people become old is because people require more attention during two phases of their lives. Our biological evolution has prioritized care for the very young over the very old, with respect to a limit on resources (like attention), effectively until the modern age. In some cultures, for instance, those with teeth must pre-chew food for those without, or expected members to engage in ritual suicide at a certain age.