The United States of America is a country founded on personal responsibility, which leads to personal and -- ultimately -- national freedom. Requiring people be responsible for their actions is one of the greatest jobs any government is tasked with handling. In general, laws are passed and put into place to ensure both individuals and groups are aware of their responsibilities and liabilities in certain circumstances. There are laws that regulate where cars may drive and how fast they may go. Contract law keeps parties on all sides of a contract aligned with the agreement they have made.
We have laws to regulate how much overtime employers pay to employees who work X hours beyond a 'standard' work week. There are laws to protect most aspects of our life, and to administer penalties when they are broken. Murder, theft, and drug dealing are all crimes, and all have punishments. These punishments are meted out with an overall level of fairness. However, certain crimes are punished in different ways depending on the age of the perpetrator.
In the United States, there are several differing regulations regarding the definition of 'legal adult' and 'minor'. For voting and smoking, legal adulthood begins on a person's 18th birthday. However, at least in most states, the legal age to purchase any alcoholic beverage is 21. The military will let you sign up as young as age 17, if you have parental permission. At 18, anyone may purchase a shotgun, rifle, or air gun, but may not own a handgun until age 21. Examples could be piled upon examples, but these are prominent.
Why is there such a disparate set of rules regarding adulthood? Is it necessarily true that if someone is 21 they are inherently better decision-makers, and won't drive while intoxicated? Or that if an 18 year-old has a beer he would be responsible and wait for a period of time before driving anywhere? What inherently gives 18 year-old Americans a better ability to make decisions and vote than a 16 year-old? I argue that age should only be a small consideration in determining adulthood, and that other factors should be considered.
In this country, children are required to be in school until age 16, unless certain, extraordinary circumstances have been met. Why is 16, then, not considered the age of legal adulthood? You can get married at 16, but not vote. You can drive at 16 (or earlier in some states), but not join the military. I propose a unified replacement to the current age-based adulthood definition.
In general, legal adulthood begins when a person has finished high school, vocational training, college, their GED, or is 21. For example, I graduated high school when I was 16. I should have been immediately allowed to do anything adults can do: sign binding contracts, vote, purchase and/or consume alcohol. In other words, I should have been treated like an adult. However, had I dropped out of high school and not gotten any further training, I should have had those adult privileges withheld until I was 21, or until I went back and completed those other conditions.
All privileges carry responsibility. Hunting is a privilege, with the understood responsibility of only hunting legal game, during legal seasons, and only taking legal quantities. Driving is a privilege with responsibilities extending far beyond you as a driver. Drivers are expected to maintain control of their vehicles, drive at a safe speed, maintain safe distances, and only operate their vehicles when not under any outside influence, like prescription drugs or alcohol.
Failure to follow these responsibilities results in (at a minimum) the temporary revocation of that privilege. For speeding, perhaps the temporary revocation is being pulled over and issued a ticket. For driving on the influence of alcohol, that revoking may be to take away your driving license for some period of time, only after which may you reapply to regain the privilege.
Certain crimes' punishment involves losing your right to vote, at least while in jail while other crimes' punishments can revoke nearly every privilege normal Americans enjoy due to life imprisonment or execution.
Redefining adulthood based around a combined age and achievement status would give a large incentive to American youth to finish school, to make something of themselves. Underage smoking and drinking would largely disappear if it was known that these activities were privileges bestowed upon anyone completing high school, but that penalties for participating held off that opportunity to enjoy the privilege by being held back in school or expulsion. There is a much smaller incentive to cheat the law when there is no staging involved.
To go back to the driving example, on a long stretch of open highway, it is much harder to keep your car within a 55 MPH speed limit than it is a 75 MPH limit. Many modern cars handle very well at high speeds, and 'want' to go fast. Extend this though to teenagers. If a typical high schooler knew that he/she would be able to purchase their own beer/wine/etc on their graduation day, they would be far less likely to acquire the alcohol before graduation at a party or prom.
My proposal, then, is to move the definition of adulthood to a more achievement-based system from the current age-based arrangement. Giving all of the privileges and responsibilities of adulthood at once, instead of spread out over time, is a more equitable solution. Admittedly, for some this may mean becoming an adult as early as 16, while for others it might be delayed until 21. What greater impetus can be given to American youth to stay in school and prepare for life, than to promise that as soon as they finish they can be a fully-functioning member of society?