How do people change their names? How many people change their names? How old do you need to be to do this?

How do people change their names? How many people change their names? How old do you need to be to do this?How do people change their names? How many people change their names? How old do you need to be to do this?
You would need to change file for a change or your name in court. You would have to be at least 18 to do it leagaly.