Overall structure, muscle shape, insertion points, hormone levels etc are determined genetically but you can see substantial changes in your physique for the first several years of natural training. From there on out, its a slow, uphill climb as you become closer and closer to your genetic limitations. When I first started (at about your age) I was able to put on 20+ lbs of muscle within the first couple years......most of it in the first year. BUt ive been building for 15 + years and I am still seeing changes. At this point I consider a 2-3 lb gain (lean mass) in a year very substantial.