Eighteen year old citizens have the right to vote in the USA. Since that is so, they are presumed to be adults - even if they are not actually grown up.
On the other hand, no State permits them to drink alcohol and, quite soon, no bank will be able to give them a credit card without a co-signer who must be both over twenty one and a person who will accept full liability. The implication of these laws, then, is that persons between the ages of eighteen and not quite twenty one are irresponsible children.
Many of them are indeed still irresponsible children but, if so, why do irresponsible children have the right to vote - with potentially significant consequences for other peoples' lives - but not to run their own lives even if they run those lives into the ground?
Perhaps the age of majority, and the right to vote, should be twenty one again.