I had to wait until my 40's to gain weight. In HS I weighed 125lbs (at 6'1"). Boot camp (PI) put on 17lbs in 3 months through a combination of a sadistic DI and good food. I stayed at 142 until... I don't know when. Now, in my late 40's, I weigh 220lbs. I'm lifting weights (StrongLifts) and trying to get rid of my spare tire.
I should have mentioned that it's eating at a surplus + lifting weights trying to gain muscle weight :) I think it takes as much discipline as trying to lose fat. I lost 10kgs in 2 months by just not eating enough after having my appendix removed in November 2013. A natural skinny person has as much difficulty maintaining weight as someone trying to keep fat off. It's a full time job.
