They may look like they're maturing earlier these days, and in terms of sexual development, they are (the ubiquity of estrogen in the environment is a likely contributor to that state of affairs), but their brains still take just as long to develop as they always have. The frontal lobes (the area of the brain that is responsible for impulse control and judgement) are not fully myelinized until well into adolescence. So you really can't go by appearances to determine how mature a young person is, or how able they are to form sound judgements. Would you let a ten year old drive your car on the highway?