Thon Maker's H.S. Coach: Passport PROVES He's Really 19 ... See For Yourself!

Thon Maker's former high school coach believes the NBA rookie's PASSPORT will help end the debate over Thon's real age ... so he posted it on social media.

In the days and weeks before he was drafted 10th overall, skeptics claimed the Sudan-born superstar is YEARS older than he claims to be ... and has been lying about his age to get a competitive advantage.

But Sky Smith -- one of Thon's coaches at the Athlete Institute prep school in Ontario, Canada -- believes Thon's expired passport (issued in 2009) will prove he's really 19 years old.

Check it out for yourself ... Smith says the passport was issued by the Australian government in 2009 -- when Thon was 12 years old. It says his date of birth is Feb. 25 1997.