A combination of physiology and sociological norms. Women become phsyically mature at an earlier age than men, which means that their brains reach an adult level of development earlier. There is also the effect of increased concentrations of testosterone in men (women do have trace amounts of testosterone, just as men have trace amounts of estrogen occuring naturally). Testosterone and other androgens increase the likelihood of aggressive behaviour and a lack of androgens relative to men causes women to appear more docile and therefore more "mature".
There is also a good amount of cultural conditioning involved. The old expression, "boys will be boys" is a good illustration of this. Our culture expects young men to get into trouble and be less mature and because this sort of behaviour is expected, it occurs.
ZV