John Cena -- Vince McMahon's a BEAST In The Weight Room

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.