Well, I think 18 as the age of "majority" is arbitrary, but there has to be a line drawn somewhere...
Case in point, and from personal experience:
My oldest son was quite mature, and trustworthy by the time he was 16, more so than many of his peers...
My youngest on the other hand, is almost 21, and in many ways is not as responsible as his brother was at 16...
Even though both grew up in the same environment(s) (divorced from ex-wife when they were young) and they are only a couple years apart in age...