Democrats had expected that the Robert Mueller appearing before them would be a steely-eyed, straight-shooting G-Man using his razor-sharp intellect to cut through congressional bluster.

Instead, the man who showed up to testify was a confused, uncertain, and stumbling figure who was unfamiliar with significant parts of his own report. For many, the spectacle was painful — a great American capping a storied career with an appearance that left him looking diminished, even enfeebled.

In modern American discourse, it is viewed as impolite to refer to personal qualities of public figures: their attractiveness; their weight; their clothing; and, yes, their age.

But Mueller's halting and ineffectual testimony meant that the elephant in the room could not be ignored: This was an old man better fitted to jostling for position at an early bird buffet than doing battle in the political arena.

Mueller's age was highlighted by Left and Right alike on Twitter. David Axelrod ventured that it was "delicate to say," but Mueller "does not appear as sharp as he was" when he had last testified more than six years earlier.

Right-wing talk show host Mark Levin described Mueller as "feeble." Sen. Lindsey Graham of South Carolina chimed in: "very confusing and sad." Pete Williams on NBC intoned: “The years have clearly taken a toll on the Bob Mueller we used to see."

Mueller will turn 75 next week. Joe Biden will celebrate his 77th birthday in November. Veteran Republican consultant Alex Castellanos was quick to make the connection, describing Mueller as "the Joe Biden of special counsels: he does not think or express his thoughts with the clarity he used to display."

On the campaign trail, Biden has indeed been the 2020 Democratic equivalent of Mueller. The former vice president rambles, stumbles, and shows greater impatience and frustration than he did as a younger man. Like Mueller, he often seems to miss the point of a question or fail to hear it.

Biden, who was just 29 when first elected to the Senate, went on to spend 36 years there before serving another eight in the White House alongside President Barack Obama.

While Mueller might be the only person you've heard using the word "bailiwick" in 2019, Biden's fondness for "malarkey" is also quaintly bemusing. On Wednesday, Mueller couldn't remember that it was President Ronald Reagan who appointed him as U.S. attorney for Massachusetts. In May, Biden mixed up British prime ministers Theresa May and Margaret Thatcher.

Both men seem like figures from a bygone age — because they are. Biden recently cited 1968 as the pivotal year in his life, when Robert F. Kennedy and Martin Luther King were assassinated. That was the year that Mueller served in Vietnam as a Marine Corps platoon commander and was wounded in action, earning a Bronze Star for valor.

Medicine is more advanced and life expectancy is greater than it once was. President Trump is 73, though only the most partisan Democrat would deny he seems more vigorous than Biden or Mueller. Bernie Sanders is 77 and cantankerous, but he appears significantly more robust than Biden. Elizabeth Warren is 70, though she has a lively campaign presence and looks younger.

Democrats on Capitol Hill are led by an ossified gerontocracy: Nancy Pelosi will be 80 next March; Steny Hoyer is already an octogenarian; and James Clyburn turned 79 this week. Sen. Dianne Feinstein of California is 86. The GOP's Mitch McConnell, the Senate majority leader, is 77.

But leading a country is different. In Britain, there has been controversy over civil servants leaking that they considered Jeremy Corbyn, 70, the Labour leader, not up to the job of prime minister “physically or mentally.”

In terms of the presidency, the facts are stark. Reagan, who was widely mocked for his advanced age and did eventually succumb to Alzheimer's, was the oldest person ever elected to the White House. He was 73 years and 274 days old when he won his second term. By the time he left office, Reagan was 77 years and 349 days old. His son Ron said he was suffering from dementia while in office.

If elected in 2020, Biden — who suffered two brain aneurysms in 1988 — will be 78 years and 51 days old. Reporters often whisper, though seldom write, that Biden wilts toward the end of his events. His campaign schedule is remarkably light, with many days set aside for him to "meet with advisers."

The tragedy of Mueller's appearance on Capitol Hill is that that is most likely to be what he is most remembered as, much like James Stockdale, the 68-year-old vice presidential candidate running for the Reform Party with Ross Perot in 1992. Stockdale was a retired vice admiral and Medal of Honor recipient who had spent seven years as a Vietnam prisoner of war. But he will be forever be known as the old man who began a debate by asking: "Who am I? Why am I here?"

A "senior moment" from Biden on the campaign trail, something he often seems on the verge of, could hasten an ignominious defeat that would tarnish the almost half-century senatorial and vice presidential legacy of which he is so proud.

Until now, Biden's advanced age is a subject that has been largely avoided by his main rivals, with only fringe candidates like California congressman Eric Swalwell, who has since dropped out, tackling it head on.

But Mueller's doddering testimony has put Biden's 76 years and counting firmly on the political agenda.