There’s a lot of talent assembled here; the director, Jake Kasdan (“New Girl”), keeps the relentlessly funny pilot moving spryly, and Mr. Savage makes an art of exasperation. But you can’t overstate how essential Mr. Lowe is, and how well the character is pitched to his abilities. Mr. Lowe — most recently Chris Traeger on “Parks and Recreation,” literally the most upbeat character on TV — gives Dean a well-meaning sweetness. He’s as much a victim of his delusion as anyone else; like Buzz Lightyear in “Toy Story,” he’s an action figure who believes he really can fly.

The life change in “Grandfathered” is more ordinary and its execution more predictable. We meet Jimmy, the restaurateur, working the floor of his successful nightspot, glad-handing and flirting to the sound of “Uptown Funk.” Quick as an elevator pitch, his sexy-single life is interrupted, as the show’s premise walks in: Gerald (Josh Peck), the adult son Jimmy never knew he fathered, with baby Edie, Jimmy’s granddaughter. As they say in those online nostalgia features: Wanna feel old?

There’s a flashback feeling to the whole story line, given that Mr. Stamos traveled from carefree bachelorhood to responsibility as Uncle Jesse in the ABC family comedy “Full House” over two decades ago.

And “Grandfathered” makes the expected stops along the route. Jimmy tries to jump into the role of dad, counseling the meek Gerald on his love life. He takes a clumsy-cute stab at babysitting Edie. And he meets Gerald’s mom, Sara (Paget Brewster), an old flame who doesn’t want Jimmy getting close to Gerald or to herself (which more or less guarantees that he will).

“Grandfathered” is as winningly cast as “The Grinder” — Mr. Stamos manages to be smarmy and charming at the same time — but its supporting characters need development. Ms. Brewster enlivens any cast she’s in, but Sara is pushed into the role of scold. (In fairness, Sara acknowledges it. “I’m cool,” she protests. “I watch ‘Portlandia.’ ”)

“The Grinder” also needs to give its female characters more to do. Ms. Ellis (the acerbic Waitress on “It’s Always Sunny in Philadelphia”) has what amounts to an “Oh, honey” wife role in the pilot. (Maybe conscious of this weakness, the show will introduce Natalie Morales as Dean and Stewart’s co-worker in future episodes.)

But any other person, man or woman, is bound to be a supporting player in the lives of Dean and Jimmy. In these comedies, they’re both the stars and the joke, funny because it’s true: For man-children like them, of whatever age, there will always be a second act.