If everyone had kids at 18-20, then the grandparents could take care of the grandkids while in their 40s while the parents build their careers from 20-40, then start taking care of the grandkids as the cycle repeats
Peoples 40s and 50s are their most productive years. We would be better off just letting people take 10 years off in their twenties - but most people would just party party party (what they do anyway)
And then you end up raising your grandkids instead of the kids you gave birth to. It's not something that comes without cost. And what if you don't particularly trust your parents to raise kids? I suppose you would have no idea whether you did or not, because they would not have parented you...