SERIES FINALE: In 1992, Jay Leno took over the Tonight Show reins from Johnny Carson, with Billy Crystal as his first guest. Tonight, as Leno says farewell to his Tonight throne and audience (for the second time, after a temporary step-down in 2009), his final guest is… Billy Crystal. Jimmy Fallon takes over Feb. 17, with a post-Olympics midnight start featuring opening-night guests Will Smith and U2. Under Steve Allen’s creative guidance, NBC’s late-night Tonight franchise launched in 1954, with Jack Paar taking over in 1957, and Johnny Carson beginning his classic 30-year run in 1962. Then came Leno. And then, for a while in 2009, Conan O’Brien, and then Leno again. That makes only six permanent hosts of Tonight once Fallon grabs the reins – and it occurred to me, last night, that I’ve interviewed them all over the years. Carson, only in a press conference setting, but the others, at one point or another, one-on-one. The Tonight franchise is storied, tenured and valued. Me, I’m just old.