HE WAS ONLY 20 AND FULL OF RIGHTEOUS passion when he first blew down from the North Country to slay cliché. Thin as a guitar pick and armed with a style that was equal parts Woody Guthrie and Rimbaud, Bob Dylan seemed destined to burn fast and fade quickly. But Dylan not only endured, he prospered, and last week (on May 24) he turned 50. If that is an unsettling milestone to some of his fans, it is one shared by many of his still-busy contemporaries. Among others hitting the big five-oh this year: Rolling Stone Charlie Watts, Animal Eric Burdon, the Mamas and the Papas' Denny Doherty, Neil Diamond, Chubby Checker, Gene Pitney—and those pictured below.