For adult men who are immature, I think it's more learned social behavior. The male mid life crisis youth obsessed have to act like I'm 20 forever *thing* You learn to care about certain things, you learn to act in a certain way, to take a certain level of consideration and accountability. If there *is* a measured difference in the maturity of men and women, I would guess that it's probably because we encourage women to display behaviors and possess characteristics most often related to maturity.
Pragmatism, compassion, empathy, patience, organization, etc. Women currently have higher college gpas and higher college attendance in general for a reason, and it's not because one sex is smarter than the other.
On the other side, men are told to be impulsive and aggressive, to disregard intellect and "go what's in your gut" and of course, to be players.
Similarly, many of the guys I know often refer to something some comedian once referred to as "thinking about nothing." Could be coincidence but no girl I know does this. We're always think think think think think think.
And naturally, some people just do what they do. An immature girl isn't exactly rare.