Judge Dredd is becoming one of these the hard way, due to the lack of Comic-Book Time in his universe. 22nd century medicine helps, but several stories have focused on his feeling like the old man of the Justice Department, and the decreasing number of his classmates still alive and active.
The Shade, from Starman, might count, though he's not a grandpa. He's over a hundred and seventy, looks like he's in his mid-forties, still dresses like it's the Victorian era, complete with cravat and spats. Oh, and he's the world's best shadow channeler, capable of summoning shadow monsters by the imperial ton, and a very experienced assassin, both with his skills and with swords. Not enough? He kicked a Black Lantern's ass. After his heart was torn out. With that very same heart. Immortal he may be, but add in his incredible wit, his own occasional act of heroism (though if you call him hero, he'll pound you) and his impeccable sense of fashion and you've got yourself a man you'd hate to offend. And that's without his actual powers.
Gorgo from Love and Rockets, retired hit man and self-appointed protector of Luba's family. He's around 90 and spends most of his time asleep, which doesn't stop him from taking out two armed assailants by himself. With his cane.
Wesley Dodds, the original Golden Age Sandman, trashes them all in this department. He doesn't have the benefit of magical shenanigans to keep him young like the other living members of the JSA, and yet while physically over 80 years old, still suffering the after-effects of a stroke and barely able to walk without a cane, goes out to save the goddamn day anyways, up to and including telling one of the most powerful evil wizards in the DCU to go fuck himself.
Bone: Supporting character Lucius Down, the aged innkeeper of the Barrelhaven, was once the captain of the royal guard. Outside the royal family, who have certain supernatural advantages, he's probably the toughest human character in the series.
Nick Fury, original flavor. Over ninety, looks like he's fifty, can kick your ass like he's twenty-five, and a Magnificent Bastard, to boot.
"I'm ninety years old. You know how I look so pretty? I take drugs. Special H.A.T.E. drugs. Life-extending drugs. H.A.T.E. has the best drugs. Because H.A.T.E. loves me. And I love H.A.T.E. Every day of my horrible drug-extended terrorist-fighting life."
The Guardians of the Universe from Green Lantern are this; being the oldest beings (technically only Ganthet is this, as the other Guardians had been killed by Hal Jordan and replaced by Kyle Rayner) in the universe and subsequently one of the most powerful.
"He's old. Like beginning-of-time, big bang-theory old. But don't let the red robe fool you. Ganthet could crack the planet in half with a thought."
Captain America is a World War II veteran. On the other hand, he was turned into a Human Popsicle in his twenties, and only defrosted 13 to 15 years ago, comic-book time, so he only counts if Buck Rogers does.
Ultimate Red Skull, being Cap's illegitimate son, is in his 60s at least, but is A) in his physical prime and B) doesn't really have a face so it's difficult to judge.
Jenny Sparks of The Authority, She may be as old as the 20th Century, but don't mess with her or London or she'll electrocute you back to 1945. And she looks like she's in her early 20s to boot.
The Punisher. Doesn't fully fit the description as he's been anything but passive, but he's still an Badass NormalAx-Crazy antihero, despite being over 60. Lampshaded in the first Arc of The Punisher MAX, as Microchip points out that Frank would've been a grandfather by that time, if his family hadn't died.
James Gordon, who in his 70s could take down a teen that got a drop on him with a military grade machine gun. Hell, most other incarnations of Commisioner Gordon, despite being somewhat younger, could also count.
Oliver Queen, even missing one of his arms he is still probably one of the best archers in the world.
Batman: Alfred Pennyworth, at yourservice (in most versions). Ex-SAS/British Secret Service, and the only member of the Bat-Family allowed to use a firearm (and he will use it if he has to).
More Batmen who meet this trope.
The Batman from Kingdom Come. He's wrecked his body through years of abuse and depends on an exoskeleton just to move, but still has the balls to deck a brainwashed Billy Batson, whom everyone else is shit-scared ofon a hunch. He later joins in the climatic fight scene in Powered Armour.
Thomas Wayne, Bruce's father, as seen in Flashpoint, in the alternate timeline where Bruce died instead of his parents that night in Crime Alley. He must be at least 60 but he can still toss around criminals decades younger than him, and kills Killer Croc andReverse Flash. Each time by impaling them, no less. Yikes.
Similarly, Thomas in his tenure as Batman in Earth 2.
Bor, the father of Odin and grandfather of The Mighty Thor. He was at least as strong as the combined powers of Thor and Odin and possibly could easily destroy the Earth. When Thor has to kill him, he has to punch so hard that his hammer was broken.
Buri aka Twiaz, father of Bor and great-grandfather to Thor, is the first of the Asgardian gods. Whereas Bor had kept young (about middle-age) due to being turned to snow, Buri has aged all that time. He comes off as a big man living in a cave. Despite being the oldest of a slow-aging race of gods he is still as strong as Thor, can sometimes beat Thor in a wrestling match, and has mystical powers beyond that of most other gods.
Jason Aaron's run introduced "King Thor", an aged Thor from a distant Bad Future where humanity and most Asgardians are long dead, Asgard itself is in ruins and the Earth is a lifeless dustball. He's weary, battle-scarred, and missing an arm and eye, but on the upside, he has the Odin-Force and a spectacular white Badass Beard (in fact modern-day Thor thinks he's Odin himself when they first meet), and he still takes on Galactus to stop the Devourer of Worlds finally feeding on what's left of Earth.
Great-Uncle, really, but Aloysius Crumrin, relative of Courtney Crumrin, is at least over eighty and will still fuck up the nastiest hobgoblin ever (with a cane sword!) or fight off vampires threatening his great-niece. It's acknowledged fact that even at his age, he's still the one that gets called in to handle the nastiest business that threatens the witch and warlock community.
The Superman from Kingdom Come. He is out of shape, has grey hair, and wrinkles, but when he crossed over into The DCU, he takes a punch from Heracles without flinching, and can move normally in a 100 G environment. Heracles earlier knocked our Superman through a building and bloodied his nose.
In Star Wars: Legacy, Treis Sinde, Imperial Knight. Infiltrating Dac, beating up a lot of Stormtroopers and squaring off against a Sith Lord, all in the issue of his first appearance. Has also the rather un-Imperial tendency to reinterpret or openly defy orders from the Emperor if he believes it is the will of the Force. DarthKrayt is also long over human expiry date and you DON'T want to anger him. The Mon Calamari learned that the hard way. His counterpart, deposed Emperor Roan Fel, does qualify too.
Ekuar in ElfQuest looks pretty wizened and decrepit, and he only has one arm and a wooden leg. On the other hand, he's pretty handy with his rock-shaping powers, and can wield his walking-stick as a formidable weapon (looks like wood, but it's reinforced with stone).
Turpin: Call off your goons, I'm an old man. And they'd hate to get their asses handed to 'em by an old man.
Magneto. Leaving aside his damn near phenomenal cosmic powers level of control, he can and will throw down without them, even if he won't always win. He's been de-aged at least once, but since most writers and artists tend to ignore that and write him at his original age, he's at least a good eighty.
He's also a literal grandfather.
Wolverine, who was born in the 1800s. His enemies Sabretooth and Cyber are also (evil) examples, as is his son Daken, who was born in 1946 and looks like he's in his 20's.
Also X-Men-related, Cable first came back to the present day as a grizzled 50-something, and must be around 70 by now, due to spending nearly 20 years on top of that in various futures raising his adopted daughter Hope. He's spent all that time fighting wars in various horrible timezones, relying mostly on his skills as a warrior, a survivalist, a tactician, and a strategist (plus his really big guns) whilst at the same time using most of his nearly godlike telekinesis to stop the spread of what is essentially super-cancer (which he's had since he was an infant). On the occasions when he can use his full power, it's been shown that it's cosmic-level in scope, allowing him to go toe-to-toe with the Silver Surfer whilst simultaenously levitating an island.
The Venom symbiote is one, having lived a very long time, and it is also a grandparent, having a grandchild.
At least one flashback story showed that Ben Parker, Spider-Man's uncle, could be like this. A former MP in the army, he wasn't a violent man by nature, but he was still capable of defending himself. He was not only very protective of young Peter, but there was also the unpleasant case where Peter befriended a classmate named Charlie Weiderman who was even more unpopular, but unlike Peter, purposely provoked the other teens. One day, he was chased to the Parker home by a group of bullies led by a rather mean guy named Rich and Ben intervened; Ben told them that if they wanted Charlie, they would have to go through him. They tried to, but Rich got a pretty rude awakening when Ben pummeled them. (Unfortunately for Charlie, Ben was rather angry at him too for provoking them, and after he clearly lied about it, told him he was no longer welcome at their house. Indeed, years later, Peter would very much regret ever making friends with Charlie...))
Iran "Pops" Sekula from Body Bags. He's old with gray hair, practically a dead man walking, and he still whips serious ass.
John Constantine himself, in his Vertigo series, which ignored Comic-Book Time. He still kicked ass, even though he was almost sixty (although a transfusion of demon blood meant that his aging was slowed somewhat), and his alcoholism and smoking hadn't lessened that much. His best mate Chas Chandler was a good example too; he was about the same age as Constantine, had an actual granddaughter, and lacked his friend's magical acumen or demon blood, but he still acted as John's muscle when needed.
Freedom Fighters has Captain Patriotic Uncle Sam, Anthropomorphic Personification of the American Spirit and looking exactly as old as he does in those World War II recruitment posters. He's also participated in every war of the country's existence and is as strong as the American people's faith in freedom. (If you count his age from his origin, he's slightly older than the USA; if you count it from when he became Uncle Sam, he dates from the 1870s, but either way he qualifies.)
The Uncle Sam of Earth-10, as featured in The Multiversity: Mastermen, has twenty years on the main universe's, and managed to survive for eighty years in a world where the Nazis won World War II.
Gramps from Cavewoman. Used a body enhancer to give himself superhuman strength and toughness, then turned up on a flying motorcycle to rescue his granddaughter from a gang of drug dealers before transporting himself, his granddaughter and his 15 ft. tall pet gorilla back through time to live amongst the dinosaurs.
Deathstroke the Terminator, aka Slade Wilson. He's one of the only characters in the DC Universe whose bare fighting skills surpass Batman's. And he has no problems messing with heroes half his age.
The Maestro, the corruptedfuture counterpart of The Incredible Hulk survived not one, but TWO nuclear wars, and absorbing the fallout made him even stronger at his base level than the present day Hulk. With his greater strength and combat experience, plus his total lack of moral scruples, he thrashes his younger self twice, winning the first battle by Neck Snap, and nearly winning the second with a good old-fashioned No-Holds-Barred Beatdown (the Hulk only won that round through luck and creative use of a time machine. Worth noting that the Maestro is well over 100 years old at the point in his timeline when the Hulk is brought forward to take him out.
Another elderly version of the Hulk, from Hulk: The End. He's the last surviving human after nuclear apocalypse killed everyone else on the planet. He's over 200 years old and absolutely refuses to die anytime soon, flesh-eating cockroaches and puny Banner be damned...
Whatever else you might say about him, General Thaddeus "Thunderbolt" Ross has always been this, even before becoming the Red Hulk. For most of his history he was an unpowered human who had no problem leading his troops into battle against the Hulk personally. Then he agreed to be merged with the electrical monster ZZZAX and gained superpowers so he could fight the Hulk directly, wound up dying saving his daughter's life, and then later came Back from the Dead. As Ross, he has the body of a man half his age and over 50 years' worth of experience as a soldier. As Red Hulk, he's among Marvel's top-tier superhumans (Bruce Banner once rated him as possibly "second only to me").
Blade, who was born in 1929, and, due to being a daywalker, hasn't really aged.
Black Moon Chronicles: Emperor Haghendorf is already an old man by the time of the main storyline, but he remains as always a Four-Star Badass who commands the imperial armies against all its enemies, has his personal brigade of war dragons, and is still one of the best swordsmen in the world. Authority Equals Asskicking indeed.