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