John Cena doesn't think there's an age limit on weight room domination ... telling TMZ Sports 69-year-old Vince McMahon is an absolute TERROR on the racks.
The WWE superstar was out in NYC (wearing only a t-shirt in the freezing weather) when we asked him about VM's recent Men's Fitness Magazine cover where the guy looked like an AARP Incredible Hulk.
Check out the clip -- Cena says that McMahon's biceps aren't just for show and the guy can lift like a man half his age ... and even rivals Cena himself.
Makes you want to call your granddad ... and ask him why he's slacking off.
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