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.
Anecdotal, yes. But your comment relies on a stereotype. How do we know that teenage mothers do not quickly start maturing from the point they become teenage mothers, relative to their peers? They almost have no choice.
(I say that knowing of some concrete counter-examples to what I just argued. But that's just anecdotes, too.)
http://en.wikipedia.org/wiki/Teenage_pregnancy#Impact_on_the...
(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).