Because when you turn 17/18 however old you are, you are so much more mature.
My maturity peaked at 9 or 10, it's been going down or holding constant since. From what I have observed, boys maturity peaks between 9 and 12, then they discover girls, and it starts to head down hill. By the time they get to their second year out of highschool it starts to level off (this can ofcourse be put off by joining a frat or business school). Well it levels off, and may, for some, start a steady but slow incline. Then you hit 65 and it takes a fucking nose dive into the deep end.
Of course your perceived maturity level is like a bell curve, and it is not untill you reach retirement age that the twO curves intersect.