His national political career began exactly 50 years ago, in early 1972. He was an obscure Delaware county councilman, 29 years old, a dashing, charismatic nobody. Republican J. Caleb Boggs was running for his third term in the Senate. Boggs was beloved in Delaware, a former governor, a Republican moderate who voted for the 1964 and ’68 Civil Rights Acts and helped put Thurgood Marshall on the Supreme Court. Senate Democrats loved him too: His good nature and common touch helped make the place run.

No one would challenge Boggs, but young Joe Biden looked at him and thought: “He’s tired.” By which he meant: He’s old. Boggs turned 63 that year, which seemed older then than now. Mr. Biden ran against him vowing to be an “activist” senator, who’d put energy in the state, which needed “change.” At one point Boggs was ahead 30 points. Mr. Biden had no money or name recognition, but he had “a great radio voice,” and he ran not so much on policies as on a mood—fresh, vital youth must take its place. It was the first federal election in which 18-year-olds could vote.

In the biggest congressional upset of 1972, Mr. Biden beat Cale Boggs by 1.5 points, and his great career began.

Now he is the target of the kind of critique he used against Boggs, coming from his natural opposition but also from within his own party: he’s lost his bite, he’s not quite with it, he’s . . . old.