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As shocking as it may seem given his youthful glow, Jim Parsons is about to turn 40. On Tuesday’s “Late Show,” the “Big Bang Theory” star told David Letterman that in preparation for this milestone birthday he “kicked nasty habits,” like smoking. Turns out, Parsons had been smoking a pack a day since he was 19, and at 35, he came clean to his doctor, who offered an interesting psychological remedy. What was his advice? Watch the clip above to see how Parsons finally quit.