Rally and I had a really lovely discussion about this earlier on in this thread.
Basically, to gain weight, you have to eat food that is high is fat or protein (or both) and either work out (to gain muscle mass) or slow your daily movement down to practically nothing. A lot of people eat a lot and don't gain weight because their body will just absorb the bare minimum. It does this, because it is used to using the bare minimum.
In other words, in your order gain mass, and therefore weight, you have to make your body use the nutrients you put in it. But, younger people tend to burn up their energy/mass due to being so much more hormonal and active. Once people get old and slow, their body stores their mass for later use (should times get tough).
