We need the demand (adaptive pressure), the modeling (apative pressure), the opportunity to practice (adapt pressure), and the opportunity to exercise externalizing responsibility.
In the families and tribes we evolved in, our mixed-aged group cohabitation would provide many of these pressures.
Eventually, individuals and the types of pressures they were placed under would diverge to some extent between males and females, with the domains for male responsibility being pointed outward from the group and the female domains being pointed inward.
But both would be under adaptive pressure to externalize responsibility within their domains until they reached full development.
Humans are not alone in this either; it is observable in many mammals that have long periods of parenting.
From the Elephant parade to the Grey Wolf pack, the Marmoset madness, and the Meerkat mob, the juveniles participate in alloparental care as part of their development and are often not afforded status as adults in the group until they give birth to surviving offspring themselves.
It is an evolutionary adaptation first, and social adaptations layer upon it, reinforcing and supporting it within the social structure.
As we’ve abandoned making parenthood the central focus of our lives, and shrunk every domain for individual responsibility, the social adaptation and social reinforcement for developing into fully mature humans, fathers and mothers, is deteriorating.