My brother was one of those. All through high school he tried to gain weight and couldn't even eating far more than the average person as well as a steady diet of pop, doughnuts, etc... That trend continued until he hit about 35; ate anything he wanted whenever he wanted and stayed thin. Then in one year, he put on about 40 pounds and found himself having to hit the gym to get it off and keep it off as well as majorly changing his eating habits.