How do we learn how to be parents today? We might read a book. How did Hunter Gatherers learn? They practised on the young. Would you rather fly in a plane captained by a pilot who has read a book or done 10,000 hours?
One of the worst things we do in our culture is to put kids into age cohorts. They just learn to be as stupid as all the other kids. That is not how school used to be until 1900 and it was never how kids were before school.
Here is Jared Diamond on HG child care experience: Link
A typical hunter-gatherer band numbering around 30 people will on the average contain only about a dozen preadolescent kids, of both sexes and various ages. Hence it is impossible to assemble separate age-cohort playgroups, each with many children, as is characteristic of large societies. Instead, all children in the band form a single multi-age playgroup of both sexes.
That observation applies to all small-scale hunter-gatherer societies that have been studied. In such multi-age playgroups, both the older and the younger children gain from being together. The young children gain from being socialized not only by adults but also by older children, while the older children acquire experience in caring for younger children.
That experience gained by older children contributes to explaining how hunter-gatherers can become confident parents already as teenagers. While Western societies have plenty of teenage parents, especially unwed teenagers, Western teenagers are suboptimal parents because of inexperience. However, in a small-scale society, the teenagers who become parents will already have been taking care of children for many years.