You know what's strange? Matt Damon has been in movies since before he was Bourne. Boom! That's a slam dunk. I should get paid extra for that joke. Let's not get sidetracked though. His first acting credit was back in 1988 where he had a small role as a character named Steamer in 'Mystic Pizza.' Luckily the movie doesn't take place in Cleveland or else his nickname would be Cleveland Steamer, and nobody wants that.
He has a total baby face in this movie, and is oddly a dead ringer for actor Jesse Plemons (an adult man). As an added bit of trivia, one of Jesse Plemons nicknames is "Fat Damon," which is one of the best things I've ever heard. Save that one for the next cocktail party you're at. Just don't steal my Matt Damon/Bourne joke. That one's mine!