Paul Vaughan investigates the physical changes that creative people encounter as they age. He talks with painter Terry Frost, actor Peter Sallis, conductor Charles Mackerras and writer Mary Wesley about how physical decline can have artistic compensations, and critic Louisa Bucklast considers two of the towering geniuses of the last century - Picasso and Matisse - and how illness, old age and declining sexual power transformed their later work. Producer Beaty Rubens