Suza McRae is Martian. She is a citizen (well, technically a minor by their law) of the independent Republic of Mars - in fact a loose federation of the various colonies.
Why Mars? Of all the bodies in the solar system other than Earth, Mars has the greatest chance of being able to sustain a thriving, self-sufficient human colony. Terraforming is unlikely to succeed - and many books make use of a terraformed Mars, including Kim Robinson's famous series - but domed colonies could easily sustain themselves. (There's also a book by Alastair Reynolds set on Mars. I won't say which one because it's far more amusing to have to work it out).
The Mars of Transpecial is not a terraformed Mars. It's still red Mars, but dotted with domed colonies. I admit there are some similarities between the Republic of Mars and the future Heinlein envisions in Starship Troopers. In his world, people earn citizenship by military service. Mars doesn't take it that far, but whether somebody is a legal adult or not is determined by competence, not age. A Martian child can gain an Independent Living Certificate, which brings with it the right to vote, as young as 14 - if they prove they are ready for it. Most gain status somewhere between 18 and 25. Some people never do. Those people include the mentally handicapped and learning disabled and a few people who choose to remain minors for whatever reason. Certain crimes can get one's Independent Living Certificate revoked, for a period of time or permanently. (Mars uses custodial sentences only rarely but does, although it doesn't come up in the book, hand down the death penalty for certain extreme crimes. Criminals are expected, for the most part, to work off their offenses in some way).
Why is Suza from Mars not Earth? Truth is, that wasn't a strongly conscious decision, except that I wanted to set as much of the book as possible off of Earth. However, I also felt that the Mars of my future - a world that, having recently gained independence in a harsh war (mentioned by many characters and coloring the story of the book) was a better homeworld to produce a self-assured yet frustrated young woman than the Earth I envision.
Besides. Mars offers our best chance to get a few human eggs out of the Earth basket. Just in case.