What a nice young man.

Jimmy Fallon, the new host of “The Tonight Show,” introduced himself to viewers on Monday like a freshly licensed doctor taking over a retiring gerontologist’s practice.

Mr. Fallon didn’t smirk or make sarcastic asides. He gently and earnestly explained that he is 39, lives in New York City and has a wife and a new baby. He pointed out his parents in the studio audience and also how the monologue works. He choked up a little when talking about “The Tonight Show” legacy. “I just want to do the best I can,” he said. “And take care of the show for a while.”

Even with celebrity cameos (Tina Fey, Stephen Colbert, Robert De Niro, among many others) and high-profile guests like Will Smith and the band U2, Mr. Fallon’s debut was more sweet than sassy. He was the grateful heir, the eager freshman, the class clown with top grades and a good heart, someone older viewers can embrace without fear of being mocked or overlooked.

And that was interesting, because the most fateful generational shift isn’t between baby boomers who were loyal to Jay Leno, the previous “Tonight” host, and younger viewers who are more comfortable with Mr. Fallon, a “Saturday Night Live” alumnus who incorporated tweets and Internet skits when he took over “Late Night” in 2009. Now it’s “Tonight” vs. tomorrow: This old-fashioned kind of programming is at the mercy of a changing landscape where audiences are balkanized, and viewing habits are radically altered.