Humans are unusual: we live long enough to be grandparents. It is suspected that this is because we evolved for them to be an important part of raising the next generation.
Working with the grain on this rather than against it, it wouldn't matter that new parents were kinda noobs at adult life, so long as they were actually adults.
Yes, I read that some scientists believe that the reason women go through menopause and can longer have kids, and also lose their ability to attract male attention (relative to younger) is so they can focus on grand kids
The problem is that human being used to live in family clans, and the western state institutions deliberately pushed the nuclear family as family clans were powerful.
And then as cities became popular for jobs, kids moved away from family.
And presently the parents buy a house and raise kids, but with ever increasing house prices and gentrification many can't afford to then live down the street from their parents.
Why "obviously"? If they're not old enough to start families, why is the age of consent that low in many places?
(When I was 16 and living in the UK, it was 16, but back then so too was the age of the end of mandatory schooling).