[1L]

All of the examples you gave involved non-adults.

Reread. The voting age involves adults. Further, it doesn't matter. My point was that different situations call for different solutions. Where in any law is an "adult" defined? Why are you assuming that if one has the right to vote and enter into a contract, he is an adult, but one that has the right to drive is not (keep reading before you respond)? Yes, adult is generally accepted to mean 18 or over, but the law itself doesn't make any distinction. It simply says: drive at 16; vote and contract at 18; drink and buy a handgun at 21. If you disagree with those ages, that's fine, but it would be arbitrary to say there should be a distinction between driving and voting age, but none between voting and drinking age.

[truenospinzone]

All states have specific statutes defining the age of adulthood, or "age of majority". It isn't just an "accepted standard". At the age of 18, the federal government no longer places on parents the obligation of support. At that point, "adulthood" is an established fact, by law. The driver's licence issue you bring up even supports this - at sixteen, you are allowed to drive, but your parents are obligated to provide insurance for you. You are still considered a "minor".