In my 20's I was completely certain that I would be successful without hard work as I was smart & talented. After a decade of that imagined success not materialising I am far hungrier, more determined and working harder now than I have ever been before.
Right now, I'm working my ass off to do a software craftsmanship apprenticeship. It has taken time to develop the maturity to be content to focus on what I need to do today and not worry about when, where, and how the hard work will pay off.
That made a lot of sense to me. No doubt we can override it but...this kind of predisposition does place restrictions that one may not otherwise account for.
Plus, what better time than NOW!
I say this as I study for midterms
But yeah, most of it IS probably driven by culture. That said, I'm sure there are some biological intricacies too. Something like x years after puberty your brain starts to want more of the same things - something that would definitely help in establishing a stable lifestyle and child rearing.