Hello again. I have another question, this time about my weight. I've heard that in your late teens you begin to "fill out" and gain weight. Seeing as I'm in late stage 4 at 16 years old, when do you think I'll probably start to fill out? And how much weight will I gain approximately and in how short of a time?
When you are in stage four, your body is able to build muscle. You can get strong when you were younger, but there was essentially a peak to how strong you could get because all you could do was strengthen your existing muscle fibers. Beginning in stage four and continuing for most of your adult life, you are able to increase the number of muscle fibers in your body through exercise. The amount you gain will be completely dependent on the amount of exercise that you do. Unfortunately, it is not a one-way street. If you stop exercising or decrease the amount of exercising you do, your body takes apart the extra muscle fibers.
Therefore, with exercise you can build up big biceps and thighs. You can also get "washboard" abs, but those won't be seen unless your overall body fat is low. Body fat ruins muscle definition by filling in the valleys between muscles.
When you reach stage 5 there is a different problem. You stop growing, but a lot of guys continue to eat like they are still growing. Thus they fill out -- around the middle -- with fat.