the reason that younger kids dont gain as much has to do with the super fast metabolism. They just cant eat enough to gain!
I think that genetics has a bigger role on the body compared to working out when you are younger. I worked out when young and was very active, but never got a "solid" muscular base to work with. I have very tight muscles, but when I dont work out they get very small.
Having said that, working out when young will affect the rest of your life style, and how much of your genetic potential is fullfulled. But, there is no reason that a person cant work up to a similar fitness level starting at 25 instead of starting at 14. It will just take much longer to get there.
Interesting thought, if I had been lazy and gotten fat with a slow metabolism from sitting around as a child, would I gain muscle easier than I do now with my fast and active metabolism?