I couldn't gain a pound of weight no matter how much crap I ate, right up to about the age of 25. So I ate anything at any time, all the time. I then proceeded to gradually gain 40 pounds over seven years, from 150 to 190 (at a 6'2" height), as my metabolism slowed.
I lived off of sugar, all the time. And pasta carbs.
You'll find that some of the formerly skinny types that keep up that rubbish diet, will end up remaining slightly thinner than most except for some out-of-place looking weight gain in the mid section. That's the lucky scenario. The unlucky scenario is you give yourself diabetes or your metabolism really lets out and you just explode to twice your size.