While the Democratic candidates seem to genuinely get along, it was inevitable that eventually they would grow more explicitly critical of one another. Which isn’t a bad thing — voters are facing a choice, and need to know what’s good and bad about their options. Up until now, however, the criticisms have tended to be vague and oblique, without mentioning their targets by name.

Which is why it is notable that Beto O’Rourke just went right at Joe Biden in an appearance on Morning Joe, saying that “you cannot go back to the end of the Obama Administration and think that that’s good enough”, and adding, “we can’t return to the past; we can’t simply be about defeating Donald Trump”.

Asked “So is Joe Biden a return to the past?”, O’Rourke didn’t hesitate at all: “He is. And that cannot be who we are going forward. We’ve gotta be bigger, we’ve gotta be bolder, we have to set a much higher mark and be relentless in pursuing that.”

This is a critique many people on the Left side of the Democratic Party share about Biden, that his goals are too modest and, rather than offering a vision for sweeping change, he essentially wants to turn the clock back eight years and continue the Obama era.

But there’s a subtext to O’Rourke’s comments, the implication that Joe Biden may be too old to be president. At least some Democrats are wondering, as Politico reports:

Donald Trump isn’t subtle about telegraphing what he believes is his best line of attack against Joe Biden: the Democratic front-runner’s advanced age.

But Democrats are beginning to publicly talk about it themselves.

In overt and indirect ways, questions about the former vice-president’s age and vigour are increasingly surfacing within his own party, fuelled by the former vice-president’s relatively light campaign schedule and attempts to limit his public exposure. If elected, Biden would be 78 upon entering the White House — making him the oldest president to take the office.

We should note that Biden is only the second-oldest Democrat in the race. But for some reason, age hasn’t been much of an issue with Bernie Sanders; perhaps it’s because he looked 77 even when he was in his forties, and hasn’t changed a bit in the time since.

For now, the oldest president in American history is Trump, who is just four years younger than Biden. And the truth is that it’s awfully hard to tell how much Trump’s age has affected his work.

In fact, Trump has proven that while doing the job of president well may require a great deal of stamina and intellectual sharpness, just doing it at all really doesn’t. Trump spends hours every day watching idiotic Fox News shows like many an angry 73-year-old, and is far more concerned with picking the right paint job for Air Force One than solving thorny policy problems. According to leaked schedules, he works a seven-hour day, with plenty of “executive time” along the way for watching TV.

Furthermore, pretty much every time Trump gives an interview or talks off the cuff at a rally, there are portions where he says bizarre, nearly incomprehensible things. Some believe that his tendency to do this is worse than it used to be and is a sign of mental decline. It might be, but it’s hard to be sure.

And if you could get in a time machine, go back 20 years, bring back a younger Trump, and put him in place of the present Trump, would the results of his presidency be any different? Probably not. All the things that make him who he is — the authoritarian impulses, the utter lack of ethics, the petty vindictiveness, the corruption, the ignorance — they were there all along.

But perhaps it is only because Trump’s awfulness is so encompassing that any age-related decline seems trivial in context. It might not be trivial with someone else. Late in his presidency, Ronald Reagan may have been showing early signs of the Alzheimer’s that would eventually take his life; even some of his own aides were worried enough about his mental state to consider invoking the 25th Amendment to remove him from office.

To his credit, Biden tries not to show offence at questions about his age.

“People have a right to question all of our ages,” he says. “It’s a totally legitimate thing. All I can say is watch me.”

While Biden’s cultural and political touchstones may be outdated, to this point he has not shown much evidence of age-related mental issues.

There are polls showing large numbers of voters expressing unease about a president who is too old, but it is one thing to ask that question in the abstract and another to make a judgment about a particular individual. Fortunately, there are few better tests of someone’s vigour than a presidential campaign.

Running for president requires long hours, extended time in high-energy situations — giving five speeches and talking one-on-one to hundreds of voters is a lot harder than spending the day at a desk — and from time to time having to deal with crises that threaten to destroy your lifelong ambitions. If Biden’s age is going to be a problem, the campaign will probably reveal it.

• Paul Waldman is an opinion writer for the Plum Line blog