IT'S NO SURPRISE that Chuck Berry has managed to slip gracefully into middle age without changing his music or his image to any great extent. Where the Rolling Stones have barged through their career doing everything to the hilt, Chuck always had that sly, above-it-all composure, and a certain subtlety about everything he did. A sense of humor too, and a smile always playing about the corners of his mouth. He never took himself too seriously. So as age crept up on him, he didn't despair. He kept doing the same thing, toning down to fit his comfort, while increasing the emphasis on what was always his prime attribute anyway: his personality. And interestingly enough, Chuck Berry as the grizzled old lecher of rock became a more commercial proposition than he had ever been expected to achieve for a second time. The Stones of late have seemingly striven for this effect too by using profanity in their songs, but where Berry is appealingly suggestive, they are merely lewd.