Watching Jay Leno say his final good-bye last Thursday night as the 22-year-host of “The Tonight Show,’’ I couldn’t help thinking of Tim Duncan. Leno and Duncan are still bringing it, on top of their respective games in their respective vocations. But both also have been told to make way for the next demographic group, er, age-appropriate successors. The departures were involuntarily. Leno is rightfully peeved about being forced to surrender his microphone, leading the ratings in his time period and still