If one wishes to join the military, the age of adulthood is 18.
If one wishes to buy alcohol, the age of adulthood is 21.
If one wishes a driver's license, the age of adulthood is 16.
If one wishes to buy a rifle or shotgun at a sports store, the age of adulthood is 21.
If one wishes an abortion, the age of adulthood is 16.
If one wishes to vote, the age of adulthood is 18 [although there is a recent push to lower the age to 16].
In the Affordable Care Act, the age of adulthood is 26.
Is it maybe a good idea to have a settled age of adulthood in the United States?