Jazz has been around long enough for us to see it's most respected practitioners become gray, paunchy, and venerable. The precocity of a Wynton Marsalis already famous by the age of twenty-three is startling. But during the swing era, youth was the norm and the advent of middle age a potential symptom of stagnation.Nearly all the prominent jazz musicians active in the 1930's (with the notable exception of Duke Ellington, born in 1899) were born in the twentieth century. Most had establis …