Tom Cruise was about to turn thirty-one when “The Firm,” the film adaptation of John Grisham’s best-selling novel, hit theatres. In the movie, Cruise plays a hotshot boy wonder just out of Harvard Law, who takes a job as an associate attorney at a law firm that turns out to be washing money for the Mob. In a pivotal scene, he has a meeting with the firm’s fixer, played by Wilford Brimley, during which he is blackmailed with a series of incriminating photographs showing his character having an affair. The meaning of the scene is made clear by the disparities between the two men: Cruise, a young man, being let in on the slimy truths of the real world by Brimley, an old man. Later in the movie, Cruise beats Brimley up with a leather briefcase.

When “The Firm” came out, Brimley was fifty-eight years old—plodding, portly, with a gray walrus mustache, his grandfatherly mien in this case turned sinister. He was just two years older than Cruise is now, appearing boundlessly vigorous as the super-agent Ethan Hunt in the latest “Mission: Impossible” movie. Cruise has been very, very famous for the past thirty-five years, and in that time it’s been difficult to reconcile his unchanging appearance with the flipping pages on the calendar. It’s easier to track his movements through the six “Mission: Impossible” movies by his haircuts—short in No. 1, long in No. 2, short again in No. 3, etc.—or the size of his cell phone, than by changes to, let alone diminishments in, his face or body. He fixed his teeth years ago; he’s in better shape than ever. And so we are left to search for other ways to keep track of Cruise’s allegedly advancing years. Thankfully, the release of his latest summer blockbuster has resurfaced one of the surest methods: comparing him to his old scenemate Wilford Brimley.

This meme seems to have had its beginnings in 2011, when Cruise turned forty-nine, the same age that Brimley was when he began filming his role in Ron Howard’s movie “Cocoon,” from 1985, a kind of “E.T.” for the olds about a group of seniors living in a retirement community who are given the chance to live forever by leaving Earth on an alien spaceship. In the years since Cruise blasted through what I’ll call the Brimley Barrier, people online have continued to make the comparison between the two men, citing the fact that Cruise was a year, or two, or three years older than Brimley when he starred in “Cocoon.” Last month, a new tweet on the subject drew the attention of Brimley himself, who retweeted it and said, “This is still hard for me to believe.” Brimley, who is eighty-three, has settled into a late-life role as a meme machine, known among a younger generation less for his years of acting than for his role as the Quaker Oats pitchman, or for his pronunciation of the word “diabetes” as the television spokesman for Liberty Medical.

In “Cocoon,” despite being reinvigorated by extraterrestrial intervention, the only athletic feat Brimley could manage was a boisterous cannonball off a diving board. Photograph by 20th Century Fox Film / Everett

It’s hard to imagine Cruise ever talking about Medicare eligibility. On his way to sixty, he is literally still kicking (and punching and jumping and hanging and remembering his long-ago co-stars’ birthdays), and doing so in a way that seems to go beyond the commonplace celebrity cosmetic trickery of the moment. He seems, in ways both inspiring and unsettling, to have figured something out. The comparison of Brimley and Cruise in middle age doesn’t just make light of the former’s premature fogeydom and the latter’s eternal youthfulness; it also highlights how the mores, signifiers, and very science of aging have changed—that sixty is the new fifty, which is the new forty, and so on. That Cruise is the new Brimley.

But in “Mission: Impossible—Fallout” there are a few fleeting signs that Cruise may be somewhat aware of the concept of aging. Even with the help of Henry Cavill, who, at thirty-five, is a year older than Cruise was in the first “Mission: Impossible” movie, Ethan Hunt cannot subdue a foe in an elaborate bathroom brawl. Later, Hunt pauses, a bit daunted and out of breath, before tossing an office chair through a window and then jumping out of it. And Hunt is succumbing to at least one unfortunate aging-man foible. As he grows older, his love interests are getting younger. Hunt’s ex-wife is played by Michelle Monaghan, who is forty-two; his new partner in love and mayhem is played by Rebecca Ferguson, who is thirty-four; and a likely contender for his affections going forward is played by Vanessa Kirby, who is just thirty.

Yet one needs merely to watch the latest movie’s stunning dénouement—a helicopter chase in Kashmir—to be reassured of Hunt’s still sure chokehold on father time, and to be reminded of the uncanny distances that Cruise is travelling beyond Brimley. In “Cocoon,” despite being reinvigorated by extraterrestrial intervention, the only athletic feat Brimley could manage was a boisterous cannonball off a diving board. Later in the movie, weighing the opportunity to leave with the aliens, Brimley’s character explains to his grandson, “We’ll never be sick. We won’t get any older. And we won’t ever die.” He decides, in the end, to go. Cruise, meanwhile, seems to have achieved the first two parts of that trifecta right here on Earth. And as for the last one, who’d be willing to bet against him?