I've been reading many articles saying that the Stones are too old, they should pack it in, look how ugly they are, they have enough money, Rock and Roll is for kids, etc...

The reason is simple. They love it. As a musician there is no greater thrill than performing music in front of a live audience. The energy and excitement is powerful. The immediacy of such electricity is exhilarating. After my first tour I went into a mild depression for a few weeks because I was so locked into that power. Music is not like athletics where there is a certain window of time that you can compete with other athletes because the body doesn't perform as well in younger years. Music is timeless. With age a musician only gets better. I often wonder what music Mozart would have composed if he had lived longer. I saw Segovia give a recital at 90 years old and he was still pretty good.