It's well established that boys and girls emotionally/intellectually mature at different rates. Generally, it's said that girls mature more quickly. And I certainly always felt that way. Growing up, the girls my age always seemed more put together than the boys.
Warning: blatant sexism follows.
I wonder, though, about the cause.
Premise 1 -- There are two broad categories of motivations -- subjective and objective. Subjective motivations are those social norms and expectations we internalize from our environment. Objective motivations are things in the external world (aside from the expectations of other people) like rocks, motorcycles, and corporations.
Premise 2 -- Girls tend to be (but are not completely) more subjectively motivated than boys, and vice versa.
I think that subjectively motivated people tend to mature more quickly up front, because they attune to the expectations of their peers and parents, internalize them, and emulate them. However, I think they plateau at some point, because they do not challenge and question the objective nature of things and their own behavior -- they do what's expected, and fail to challenge it.
I think that objectively motivated people tend to mature more slowly up front, because they are not attuned to expectations. They don't particularly care what pleases others, because they are focused on things. However, I think that in the long run, these people tend to continue to mature long after the subjectivists plateau, because once they develop behaviors through trial and error, they grasp the essence of the reasoning behind those behaviors, and continue to develop.
A girl learns that people respond positively to you when you dress pretty, so she dresses pretty. However, she also judges people who don't dress pretty, because they aren't doing what's expected.
A boy doesn't notice or care how people expect him to dress, and so he dresses poorly. He also doesn't judge people that dress poorly, because he's attuned to the essential -- it's what's inside that counts. After a few years, however, he learns that if he wants to succeed in a career he likes and make money he wants, people will judge him if he does not look professional. As a result, he begins to dress better when necessary, but without being pretentious about it (recognizing that it is merely cultural), and without judging others. Thus, more slowly but ultimately in a superior way, he learns the essence of how and why to dress well.
Applying this model to a broad generalization, I'd say this might explain why girls (who tend to be more subjectively motivated) mature more quickly than boys up front, but in mid to late adulthood, men surpass women in maturity.
As always, this does not necessarily apply to individuals. I know girls who are objectively motivated and boys who are subjectively motivated, and I've seen boys plateau and girls surpass. Also, some boys are more objectively motivated than other boys, and the same with girls. Even so, the application of this causal mechanism to explain the general trends of maturity would still reflect itself in gender maturity rates, due to the general tendencies of the genders.