One of the nation's top art critics shows how six great artists made old age a time of triumph by producing some of the greatest work of their long careers--and, in some cases, changing the course of art history. Ordinarily, we think of young artists as the bomb throwers. Monet and Renoir were still in their twenties when they embarked on what would soon be called Impressionism, as were Picasso and Braque when they ventured into Cubism...