Former “Tonight” show host Johnny Carson turns 75 today, but since he retired from his nightly duties eight years ago, the suave Nebraskan has kept good to his word to keep a low profile. “He was able to say good night, and that was the end of it,” his longtime sidekick, Ed McMahon, said this morning on the “Today” show. Insult artist Don Rickles (who is all of 74) told “Today”: “Now that he’s 75, I realize: He’s old.” His birthday is reportedly being celebrated quietly, but Carson is not totally out of the picture. In the new issue of The New Yorker, out today, his first short article appears. It is called “The Proverbs of Dennis Miller,” and it pokes fun of the new “Monday Night Football” commentator. Happy birthday, Johnny, and welcome to the Fourth Estate.