 Oddly enough, we're not really sure why tendons around your body degenerate, but they probably has something to do with the fact that the cells and the tissues and things all mature and they just don't take the amount of stress that they took before and also just don't heal as fast as they did before. 30 to 40 is about when things start to unravel a little bit and it's very common, not only in big athletes, but in those of us who try to stay in shape. There are a lot of non-athletic people who seem to get shoulder problems even though they haven't really done very much.