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.
At 18 most people will have just come out of a regimented education system, never had to pay bills etc