How Come Artists Lose their Skills?

Musicians, Writers, etc... How come so many artists lose their skills when they get older? For example, I used to like Stephen King books, but for the last 15 years he has just been putting out crap. I like many music groups, but in case after case, I prefer their older stuff (when they were just starting out) compared to their newer music! So many of them were considered "geniuses" in their day, putting out great stuff, and then they stop. Stevie Wonder is a good example. I liked U2 up until 1992, since then the music just doesn't have the "spark" for me. The same with The Cure. Does age kill the creative genius?