The most common age of majority in the United States is 18. Almost all states have this as the age at which a young person officially becomes an adult. At the age of majority, a person is seen as an adult. They are no longer automatically seen as the dependent of some other person. They are typically able to do things like entering into contracts. We should note that a person can reach the age of majority but can still be unable to legally do certain things. The most obvious example of this is the right to drink alcohol. People are considered to be adults at age 18 but cannot drink alcohol in the US until they are 21.
My own view is that 18 is about the right age. I think that the idea of an age of majority is an impossible idea. People do not mature at the same rate. There are many people who are only 16 years old but who are more mature and responsible than other people who are 22 or 23. It is absurd to think that we can set an age that can accurately specify when all people become responsible adults. However, legally speaking, we have to do this. Because we have to set the line somewhere, I think it is logical to set it at 18 since that is about the age when most people finish high school and are likely to set off more or less on their own.