The legendary actor -- whose legend only continues to grow with each new action flick he's cast in, including his latest for Netflix, "The Ice Road" -- did an interview with EW, in which he dissected the phenomenon of him getting tapped to play the hero time and time again.

He says, in part ... "I just turned 69 years of age, and the conversations I have with my agent now are, 'Liam, have you read this script? It's an action script,' and I say, 'Chris, let me ask you this, they do know what age I am, right?' 'Yes, they do.' 'Okay, that's all I want to know, thank you.'" So, yeah ... everyone seems to be in on the secret -- he's a senior citizen!

Liam adds, "That being said, I keep reasonably fit, and you just have to for some of these films. If you're playing the lead in any film, you have the responsibility; you're in practically every scene, it behooves you to be fit. You don't have to look like Arnold Schwarzenegger at the age of 35, but you have to have stamina."

Of course, Liam as the modern-day Stallone/Willis/Schwarzenegger can largely be attributed to his standout action film, "Taken," which became a cultural hit ... and apparently led to Hollywood honchos thinking LN would be THE perfect guy to whoop on baddies going forward.

And, hey ... the dude's not complaining either. Seems like he's taking this bizarro typecasting in stride -- but he'd be far from the first mainstream actor to be playing an older gun blazer.

Yes, we're aware Harrison Ford is once again donning the brown fedora for "Indiana Jones 5" -- and he's 78, so he might be the king in this convo -- but the one and only Charles Bronson seems to have been one of the early few to pave the way for both HF and Liam.

The man was acting in action flicks -- including a fifth installment of 'Death Wish' in 1994 at age 72/73 -- until he was in his late 70s, before passing in the early 2000s at age 81.