He dropped out of school at seventeen to join the Army so his father couldn't make him go to Cambridge university to teach mathematics, but by this point he'd already earned his degrees (3 Ph.Ds) and any work he had left was just tidying up course work. He (technically) left a week before his seventeenth birthday, but he thinks of it as his seventeenth birthday.

UPDATE 03/10/2013 - This makes Tyson's timeline, at this point in time, read as follows: seventeen year in Cambridge, eleven years in the army (four years basic training, followed by five years training the dogs, and then two years on the front line with Sebastian), fifteen months on the run with Sebastian, four months in the Middle East, two months travelling across Europe, and just over a year and a half with Jim.