My perspective is that life circumstances often force you to "get your shit together." Specifically, having a spouse and kids to support provides an instant shot of maturity and focus for most (decent) people. That was certainly the case for me after having my first child at 26. Since more educated adults these days seem to be delaying marriage and children into their early thirties, it naturally follows that more twenty-somethings would lack the motivation to lead a more disciplined/career-minded lifestyle.