In a way, you're right. We set the age of adulthood at 18 because we've determined it to be the best balance between peak physical prowess (especially with regards to the Armed Forces) and emotional maturity, even though very, very few people are truly mature at that age. We also amended the Constitution to allow 18-year-olds to vote - if a kid is old enough to pick up a M16 and kill people with it under military orders, why isn't he old enough to vote for the folks sending him to war? The age of consent is 15 or 16 depending on the state you live in. At some point we decided that it generally wasn't a good idea for 14 year olds to legally be able to engage in sexual relationships or make such a permanent decision as marriage. You can't run for POTUS until you're at least 35 years of age, and even at that, a 35-year-old is not going to be seen by the general public as being experienced enough to lead the country. But by and large, teenagers are still not equipped to deal with life on their own. Some of the ones most easily preyed upon are the ones that seem the most "mature", because their false sense of confidence leads them into situations that they get trapped in.
At the time he became a general in the Union Army, Ulysses S. Grant, later the 18th President of the United States, had been living with his parents at the age of 39 because he had been unable to make a living on his own after being forced to sign a letter of resignation from the Army several years earlier for showing up for inspection drunk, as an alternative to a court-martial. When he got his commission as General, his father wrote him and told him that now that he had a good-paying job, he'd better keep it!
