You do not have to be an adult to have your work published. There are plenty of minors who have their books on the market. However, you do have to have a parent or legal guardian willing to represent...
It has language and it's hard for a kid to understand, but I would say an 9th grader and up might be able to read it.
Book publishers publish books. Here is a link to give you some of the names of different book publishers.
Phillis Wheatley. Please see the related link below.
More or less, yes.