If people say they they eat a lot and don't exercise or gain weight, its most probably because they are overestimating the amount they eat. Then again there are other factors involved - age, sleeping, health stress etc.
While at college i used to train with weights, a lot. I used to think being big was cool but i could never find any decent fitting clothes and the extra muscular weight played havoc with my knees (although i didnt realise it at the time).
Weights are still an important part of my routine but i do a lot more cardio nowadays and i don't go heavy on the weights.
Light to medium resistance training is very good when you are young as it strengthens your bones which will help when you are old. It is also very good at lessening the effect of osteoporis in women.