Senility is becoming an overt line of attack for the first time in a modern U.S. presidential campaign. Why it matters: As Americans live longer and work later into life and there's more awareness about the science of aging, we're also seeing politicians test the boundaries of electability. Biden is 77; Trump, now 74, already is the oldest person to assume the U.S. presidency. Driving the news: As President Trump ramps up insinuations that his general election rival is doddering, Joe Biden turned the tables on Tuesday, saying Trump “doesn’t seem to be cognitively aware of what’s going on” with his own briefings about Russia and U.S. service members.At the same news conference where he took a swipe at Trump, Biden was asked by a reporter if he has been tested for cognitive decline."I've been tested and I'm constantly tested," Biden responded, adding that "I can hardly wait to compare my cognitive capability to the cognitive capability of the man I'm running against."A Trump campaign Twitter feed played back the clip and asked, "Did Biden take a cognitive test? What were the results? Why is he getting frequently tested?"Biden campaign advisers tell Axios' Alexi McCammond and Hans Nichols that the testing Biden was talking about is the past 15 months on the campaign trail and that they see Trump's attacks, in psychological terms, as "projection." They also said Biden wasn't previewing a new theme, just highlighting Trump's attempt to distract from his own vulnerabilities.Flashback: Candidates’ age and mental state have been questioned in past presidential campaigns — remember Barry Goldwater in '64, Ronald Reagan in '84 and Bob Dole in '96 — but never like this.The big picture: Cognitive decline is a problem many older Americans deal with and a legitimate question for presidential candidates to be asked — and answer — says historian Julian Zelizer. “We have a race right now where we have two old candidates, that's just empirically true,” he says. "I think this presidency has shown us that somebody's mental state really matters.""With each of these candidates, the question has been raised — Trump because of everything he's said and done" and remarks that often are "just a mishmash of words." "Biden, there's questions, more subtle I think, about he doesn't finish every sentence, or during the debates he'd pause and seem to have trouble thinking about what he wants to say. Those are the moments that then become questions."Between the lines: Both men's durability as public figures also invites easy comparisons of them now with their younger selves, Zelizer says. "These are two people who at least since the '80s a lot of Americans watched, almost like a family member. These people, we've seen since they were in the prime of their age, and now they're older men." The bottom line: Though they have different personalities, both Trump and Biden are known for speaking off the cuff, opening themselves to verbal gaffes and criticisms of rambling.