Regardless of whether it's a game mechanic or not it's something that Ezio does, just like how he survives several-hundred foot drops into hay bales or how Armstrong survives blows that under all laws of physics would kill him (that being story mechanics, the developers simply making him OP for no reason).
I want to point out your odd vocabulary here. When Ezio defies the laws of physics you kind of shrug it off, but when Armstrong does similar feats you call it being made OP for no reason. Maybe you're referring to the actions of both of them as OP but the emphasis is definitely on Armstrong's actions.
Also, yes, Ezio has had it for over a decade but his agility has hardly diminished; if anything he has gotten better with experience. He shows absolutely no signs of his agility diminishing but he does get new technology to make him a better assassin and his combat skill and agility improve vastly.
Please, his signs of aging do slowly come to light over the course of the game. For one, he gets the shit knocked out of him at the beginning and very nearly dies. While he survives and recovers more or less completely, through the rest of the game based on his sounds and reactions you can tell it's much more strenuous on him than it was in his youth. And the tougher enemies are now extremely difficult for him. While before the greatest foes he faced (such as Rodrigo Borgia) were difficult but still no match for him in a prolonged fight, he now goes up against enemies that can beat him within an inch of his life (The Borgia Captains. Cesare himself sucked.)
What is the toughest enemy Ezio has ever faced? Maybe a half dozen of the Papal Guard? Tough, but I don't think he'd last an instant against the kind of stuff Armstrong deals with.
Here, for reference, is the Armstrong's toughest fight.
Part 1An interim that the two-parter video missed for some reason. Sorry about the music.The second part of the two-parter is mostly a team-up showing him and buddies beating the shit out of Sloth. Armstrong couldn't beat Sloth on his own, but he did some serious damage and remained on his feet, which attests to his endurance.
The reason I bring this up is to show that a bullet isn't going to put him down, nor is one knife, no matter where you put it. Before he goes down Armstrong's going take Ezio with him. And that is
if he goes down which I still strongly disagree with.