Our big screen tough guys are fading into the shadows, right before our eyes. A generation of them are either hanging it up or descending into C-movies to pad their estates as their years of kicking ass and tearing off tough one-liners pass.

Liam Neeson leans into his 70th year with a thriller tailor-made for a hard man “they” won’t let retire. In “Memory,” he wears the years and the miles on his weathered face and no-longer-hulking frame. He plays a hitman who understands that it’s not the aim, the eyesight and the physique that decline the fastest, it’s the mind.

In this adaptation of a 2003 Belgian thriller, Neeson is Alex, a tall older gent who travels the bloody borderlands between Texas and Mexico, driving inconspicuous beaters, dropping burner phones, passing himself off as a nurse, when the occasion arises.

That’s how he gets access to a “contract,” a made man visiting his mama in the hospital. Unsentimental Alex garrotes the guy, right in front of ventilatored Mom. Not his tidiest job.

“I’m getting out” he tells Mauricio (Lee Boardman), his longtime go-between. “Men like us don’t retire,” Mauricio reminds him.

There’s another job — a two-target hit — in Alex’s old home town of El Paso. He’ll do it, reluctantly. And he’ll check in on his catatonic brother in the nursing home while he’s there.

Guy Pearce plays a similarly grizzled Federal agent who just saved a young teen (Mia Sanchez) from her sex trafficker father, but who resists his efforts to put her in a decent foster care living situation. She’s a kid who “knows things.” Unbeknownst to Alex, this tarted-up photo that he is told is his second target is her.

“She’s a child! I won’t do it!” doesn’t get a sympathetic hearing. So Alex takes on a new mission. He’ll evade the Feds and local law enforcement (Ray Stevenson) and take out all the mobsters in this Mexico-to-Texas human sex trafficking/drug trafficking cartel, one by one.

Director Martin Campbell, of “Goldeneye,””Casino Royale” and “The Mask of Zorro,” more lately behind the camera for a Jackie Chan’s last hurrah “The Foreigner” and Maggie Q and Michael Keaton’s “The Protege,” ably sets up our quest, the moving parts in it and the story’s hook.

Alex is on medication. He’s losing his memory. He has to write details on his arm, in magic marker, just like Guy Pearce’s character did in Christopher Nolan’s “Memento.”

Is he still nimble enough and tough enough, can he remember enough to pull off this house cleaning?

“We all have to die, Vincent,” he tells his FBI pursuer at one point. “What’s important is what you do before you go.”

Neeson’s distinct, growled gift to one-liners is given a nice workout in Dario Scardapone’s script.

“It wasn’t supposed to go this far!” a target pleads.

“If I’m here, it’s gone too far!”

Of course there are stunts and physical feats that would be hard to believe a guy 20 years younger could pull them off. And there’s a moment, when Pearce and Neeson share a tense scene that goes over-the-top memory-loss loopy that we can see “Man, can you hear the ridiculous lines they gave you?” in Pearce’s eyes, staring down Neeson.

That’s a sign a decent if formulaic thriller has taken a dive.

The film tries to walk a narrow political line — suggesting rampant Mexican criminality and Texas corruption and a Mexican agent’s (Harold Torres) outrage that “the appetite for big fish ends at the border.”

And there are a couple of grabber moments that punch up a seriously played-out vengeance thriller plot. Let’s just say your days of taking the exercycle right next to the window of that strip mall “gym” you frequent will end after “Memory.”

But the premise, that a not-remotely-young man suffering from the early stages of dementia could still overpower and outshoot one and all, perform self-surgery on a gunshot wound, sneak onto a yacht party and hit well-guarded targets, seems a stretch. As is the Hollywood way, his illness picks the damnedest places to announce its presence. It comes and goes according to what the script dictates.

It’s still a half-decent movie, closer to Neeson’s late-career “Taken” peak than his most recent films. But if he’s letting the audience see the writing on the wall, it might be time for him to stop and read it, too.

Rating: R for violence, some bloody images and language throughout.

Cast: Liam Neeson, Guy Pearce, Monica Bellucci, Taj Atwal, Ray Fearon, Louis Mandylor and Ray Stevenson

Credits: Directed by Martin Campbell, scripted by Dario Scardapane, based on the Belgian film  “De zaak Alzheimer,” “The Memory of a Killer,” adapted from the novel by Jef Geeraerts. An Open Road release.

Running time: 1:54