The OP has a good point that is related to the larger question of, just when does a person reach adulthood? There really should be only one answer that applies in all situations.
A juvenile who commits a serious crime might be tried in court as an adult at 14 or 15 years old. Sometimes even younger. Why are you an adult in this case but in no others?
A parent may have to pay court ordered ‘child support’ until the child reaches the age of 18 yrs. Except if the child is going on to college, then they are still a child and you keep paying until 25 or 26. Unless they happen to drop out or quit college, then they are suddenly an adult again.
The new health care law includes an already active section where a parent’s health insurance must provide coverage for the ‘child’ until the age of 26. Which is a great deal for young people just starting out who can’t afford insurance, but when are they really adults and responsible for their own self, 18 or 26?
Even though there is not currently a draft for the armed forces in the US, males are still expected/required to register for selective service at the age of 18.
There are many more points of reaching adulthood that are all over the place. Age of consent to marry or even have sex legally, when to own property, etc.
The move to make adulthood 18 with regard to drinking, drafting, etc. was a brief period of sane equality that didn’t last. The trend for the future is to prolong the transition into adulthood.