Ric Flair. There's the Worked Shoot promo Flair cut on Carlito ("I'll tell you what my problem is. Guys like you that have no passion, no GUTS! You want all the money, you want all the glory, you wanna fly first-class, you wanna walk around with someone like her? You don't deserve it!"). Not to mention working a full-time schedule in his sixties and blading like a mofo more than a few times to sell a feud. Still king of the craaaazy eyes, Flair. There's a reason Flair's still on TV and Carlito isn't.
WWE Hall of Famer and wrestling legend The Fabulous Moolah qualifies as a Badass Grandma; Moolah won her last WWF Women's Championship in 1999 at the age of 76 and remained a force in the WWE's Women's Division right up until her death at age 84. Her longtime friend and tag-team partner, Mae Young, was still semi-active at age 86; by that time, she was more of a comedy character in the mold of Mae West, though. Vince McMahon famously promised that, if either Moolah or Young (or both) lived to see their 100th birthday, he'd let them wrestle a match. (Furthermore, the match would be against Stephanie McMahon's daughter, who would be 17 that same year.) However, Young didn't make it either; she died at age 90 in January 2014.
To reiterate that Mae Young deserves the Badass Grandma label as well: in 2000, during the infamous angle that saw her give birth to a hand, Mae Young was put through a table by The Dudley Boys. Twice. According to the Dudleys, they weren't sure if they should go easy on her, but Mae smacked them upside the head and told them not to hold back. Keep in mind, Mae was in her 70s when this happened.
On the Old School Raw she challenged LayCool to a No-holds Barred match and won.
More common in Japanese wrestling: some older wrestlers develop an ability known to some fans as "grumpiness", when they get so fed up with those gosh-durn kids with all their fancy moves that they just suddenly no-sell everything and knock the whippersnapper's block off with a single slap. Best exemplified by Genichiro Tenryu, though by no means is he the only wrestler who does this.
Terry Funk once commented that he thought the most hardcore man in wrestling was Vince McMahon. Terry's reasoning? On some level, the wrestlers do what they do because they need a paycheck; Vince, on the other hand, is a billionaire who will never want for money again, and could do anything he pleases with his life, but gets beaten up, thrown around, steel chairs wrapped around his head, thrown through tables, etc. on national television just because he likes it (and he knows people love to see him get beaten up, since he plays the heel all too well). Vince is also an actual grandfather, which puts him squarely in this trope in Real Life.
It's been said that there's nothing Vince won't ask a wrestler to do that Vince won't do himself.
The Undertaker certainly qualifies. Even in his forties, he's capable of putting out a good effort against guys younger and healthier than he; in fact, 'Taker and Shawn Michaels - both at least 40 years old at the time - showed up EVERYONE younger than them at WrestleMania 25.
On that note, Shawn Michaels: he's over 40, and he can still pull out at least a three-star match out of pure instinct. And this is AFTER suffering a back injury that left him on the shelf for four years and unsure if he'd ever wrestle full-time again.
The Iron Sheik. He may not go in the ring anymore, but WATCH OUT or he will suplex you, put you in a camel clutch and fuck your ass to make you humble old country way.