Earlier quoted context omitted.
I don't believe that a low birth rate says anything about when people grow up. If it did, teenage mothers would be the most responsible people in society.
I mention it as example of adults who have priorities traditionally associated with younger people - the low birth rate in places like Germany or Italy partly comes from adults preferring a certain lifestyle (a better car, exotic vacations, living in fashionable=expensive neighborhoods etc) (I'm not judging this, btw, it's a valid choice.)
In Scandinavia birth rates are relatively higher - definitely not because Scandinavians don't like cars or vacations, but rather because the society makes it easier to have children and a job at the same time (e.g. kindergartens available and so on).