The age of majority (age at which a person ceases to be a minor) used to be 21 in most US states. It was lowered to 18 by statutes in various states at various times -- the Vietnam War had a major political effect on this. As the comment by Nate Eldredge says, the right to vote at at 18 is guaranteed by the 26th amendment to the US Constitution. It is possible, but seems unlikely, for a particular state legislature to return the age to 21 by law, at any time for all rights except for voting, or for specified rights. For example, in many states the right to purchase alcohol is only granted at age 21.
If a child is incompetent in a legal sense, one can apply for Guardianship and a judge will evaluate the situation and make an order of Guardianship if the Judge believes it justified. This is normally used only for serious mental illness or disability, or other condition which makes the child (or adult ward) unable to function in society without help, and unable to make decisions in his or her own best interest. The guardian must make regular reports to the court, in most cases. The detailed law of guardianship varies from state to state.