Warhammer 40,000. Commissar Yarrick. He's not a Super Soldier, a SpaceElf or a robot zombie. He's just an old, old man with unfinished business and a giant power claw for a hand and a laser beam eye. Though everyone in the above three categories does outlive a normal human a few times over and still remains badass.
Scout Sgt. Torias Telion of the Ultramarines. He's personally trained at least three of the Chapter's Captains, making him older than many people who outrank him. Most Ultramarines say that they owe their lives and successes to his training.
Commander Dante, the Chapter Master of the Blood Angels is one of the oldest Space Marines alive, being over 1000 years old. The closest most Marines has gotten is several hundred. This makes Dante a master strategist and is still a beast on the table. He even fought and clove one of The Mightiest Daemons of Khorne, the Bloodthirster Skarbrand, in two.
Let's not forget: he's been Chapter Master for over 1000 years. His real age is unknown, though it must be decidedly greater. No one else, outside of Dreadnaughts, can remember a time he wasn't the Chapter Master of the Blood Angels.
Space Wolves tend to have quite a few of these. Their Chapter Master, Logan Grimnar, is so old that he's earned the name of "Old Wolf", and has a full-on grandpa beard made completely of grey hair. He's said to be strong enough to be able to fight a Bloodthirster one-on-one. And then there's Ulrik the Slayer, the chief Wolf Priest, who is old enough to call him "Young Grimnar."
The oldest Space Wolves Dreadnought is Bjorn the Fell-Handed, one of the few individuals who can claim to have fought alongside Leman Russ and the Emperor. Also, as a corpse locked inside a giant robot loaded down with weapons, he has a rather foul mood and the firepower to back it up.
The 13th Great Company of Space Wolves consists of ancient marines who served alongside with Bjorn and Russ himself.They have been fighting Chaos inside Eye of Terror for 10,000 yearsnote Kind of; time flow in the warp is very strange, so it could have been only a few hundred years for them since the Horus Heresy, or their fight may have lasted longer than the Imperium itself.
Eldrad Ultharan can Mind Rape any character in the game to death with a little bit of luck, is a brilliant support mage, and isn't too shabby in close combat either. He was around during the Founding of Imperium and rumored to be just about the only living being in the galaxy who knows the Emperor's given name.
The Phoenix Lords were around during the Fall of the Eldar, and (depending on which one) eat Space Marine Chapter Masters, Land Raiders, and possibly even entire Imperial Guard Regiments for lunch.
Asdrubael Vect. The guy was old enough to have seen the Fall, which would make him easily ten thousand years old and probably the only living thing older than a Chaos God. He's also a big enough Magnificent Bastard that he rose to the top of Dark Eldar society from slavehood and has stayed there for millennia, and he's an epic close-combat beatstick on the tabletop; he's one of the few special characters who can reliably take on and smack down the Tyranid Swarmlord, and squads of regular mooks suffer Critical Existence Failure the moment he gets within arms reach.
The inimitable Marneus Calgar, Chapter Master of the Ultramarines; probably around a thousand years old, fitted with bionic replacements for his four severed limbs and his damaged left eye, dual-wields Power Fists, and punched out an Avatar of Khaine and lived to tell the tale. (Arguable: Calgar is just slightly old by the standard of Chapter Masters and certainly younger than Grimnar or Dante.)
And last but certainly not least, THE EMPEROR OF MANKIND. A Physical God almost as old as Homo Sapiens, who's spent the last 10,000 years holding the Imperium together and protecting it from Chaos WHILST BEING A VIRTUAL CORPSE IN A LIFE SUPPORT MACHINE.
Jake Kramer, part of the spec-ops well past the time when he should have gotten a discharge. And kicking ass.
Sebastian Crenshaw. Picture Sean Connery playing Ryu Hayabusa, and you have Crenshaw in a nutshell. Around sixty in a setting where average life expectancy is forty, and the personal assassin of the most powerful man alive. Also a social chameleon and the life of every party.
Cardinal Dominic, an old bitter Knight Templar who has personally executed innumerable heretics.
White Wolf's epic anime fantasy game Exalted has plenty of those. To name a few most outstanding:
Chejop Kejak is about 5000 years old and will die of old age in about 5 years. He's old, tired and bitter. He's also the most powerful Exalt currently living and a master of several Kung Fu styles capable of literally rewriting reality. Not to mention he actually rules Heaven.
Well, to be more precise, he is one of the people most listened to in Heaven. He is the most senior member of the Bureau of Destiny, has very deep connections across the rest and has an Essence rating of 10. To put it in perspective, no god in Heaven other than the Incarnae have Essence ratings of 10, and the Incarnae are hopelessly addicted to the Games of Divinity, so Chejop Kejak is the most powerful personage in Yu-Shan still politically active.
Saibok Gauto's age is hard to assess, but definitely over ten thousand years. He's a feeble, senile man with more wrinkles than body hair, and the only thing he cares for is his great garden. When in danger, however, he shows superb strength and fighting abilities sufficient to beat the living crap out of any starting character.
Though he's much younger than either of the above, the events of Yurgen Kaneko's Exaltation were tinged with this. He was a barbarian warlord who realized he'd grown too old for the battlefield and, following the standards of his tribe, walked out into an ice storm to die. The Unconquered Sun had other plans, and now he's managed to unite the various barbarian tribes of the North and give the Realm hell.
In BattleTech, Clan Mechwarriors are but mayflies to the Inner Sphere, where your average 'Mechwarrior (Assuming s/he isn't killed in combat) will be in the cockpit till their 40s. But several examples of Badass Grandpas (And Grandmas) include Natasha Kerensky (In her 70s when she was finally taken down by a rare Clan 'mechwarrior almost as old as her), Jaime Wolf (died in his 80s during the Word of Blake-sponsored invasion of Outreach, after taking out an entire company nearly singlehandedly), Takashi Kurita (Who at the age of 80 had to be restrained by an agent of his son just to stop him from charging headlong into a clan invasion force of the capital world) and his son Theodore (Who fought a war on two fronts, against the Lyran Commonwealth and Federated Suns forces, and against his own army's staunch traditionalism sponsored by his father, at the age of 43, and only went further as time went on.)
Special mention should be made of Morgan Kell, who fought in his ancient Archer Battlemech into his 90s, and was the only living 'Phantom Mechwarrior' - essentially invisible to targeting computers, a condition he gained in his late 30s.
Nondi Steiner. She is 86, General of The Army of the Steiner side, and still deadly (although We Know Better).
Werewolf: The Forsaken has a whole Lodge of these, known as the Lodge of Scars. Somewhat justified, as the werewolves who quality for said Lodge have survived decades in a life where they're constantly hounded by hostile spirits, in the middle of a holy war with their bastard cousins, and always one step away from going into a killing rage.
Dungeons & Dragons characters can potentially become this if the player makes them old. Outside of this though, two specific examples come to mind: Elminster from the Forgotten Realms and this guy◊ who appeared in 3.5 Edition's DMG 2.
Some veteran players and Game Masters, especially those who do not keep Comic-Book Time between dungeons/campaigns, end up with characters that become this. Veteran players themselves, can be seen as this to younger players, in that they know very well how to break a character - because they had to try much, much harder back in the day to do so.
Monks (or at least 3.X monks) are destined to become this if they hit level 17 while still "young". They will not receive any further penalties (existing ones stay) to physical ability scores for aging, but continue to gain the increases to mental scores. As wisdom is a mental score, and important to monks, if they do hit level 17, they will be at the peak of their combat skills at an age where any other melee class would be long retired.
Druids get the same ability to resist aging two levels earlier, and Wisdom is even more important to them because they cast spells with it. Not that they're slouches in physical combat either, with the ability to Wild Shape into a variety of ferocious animal forms.
King Boranel of Breland in Eberron is facing old age and questions of who should succeed him on the throne (his kids are either not as charismatic or not as smart as him), and is around level 12 in a setting where few people top level 5. Most NPCs of at least that level are major campaign antagonists. He's a legitimately badass, well-loved war hero and has some big shoes to fill.
Call of Cthulhu has an option to pre-age your character at character generation. The harshness of the penalties depend on the era you're playing in, but by the 1920s you can effectively trade a little strength for enough skill points that you never miss with your preferred firearm type at a realistic range.
Old Man Henderson, now with his own page, stands out in particular. Very handy with a shotgun, could level a church full of cultists "with the speed and brutality of the fucking Spetsnaz", extremely handy with explosives and permakilled Hastur.
In Cygnar promoting a warcaster to General is usually a final honour before retirement; upon getting his new rank badge, General-Adept Sebastian Nemo stopped inventing cool weapons and went straight to the northern border to save the primary defense fortress from destruction. Before promotion, he invented the Storm-Chamber tech powering the weapons of the various Storm Companies, built two sets of armour based on the same power (one for him, one for his star pupil), designed and built the most advanced Warjack his factions ever seen outside the armies of Cyriss, and trained Lord-Commander Stryker, a man in command of about half of Cygnar's military power. He has gone toe to toe with the biggest and baddest warcasters of rival nations time and time again over 60-ish years of service, and will happily demonstrate the exact reason why you should respect your elders, if you push him.
Theres also Karchev of Khador. After being crippled in battle a hundred years ago he received a warjack to serve as a surrogate body and has continued serving ever since. He's seen countless battles and has more experience than anyone outside the undead generals of Cryx. Being both a warcaster and warjack he's a ten foot tall iron and steel mecha with access to magic.
Hierarch Severius of the Protectorate of Menoth is way up there in the years, but this doesn't stop him from leading the Protectorate and personally leading the Northern Crusade. Despite his physical frailty he's a brilliant tactician and leader and can invoke holy power to smite his enemies, protect his allies and completely shut off all magic nearby.
In Nomine by Steve Jackson Games, meanwhile, has Michael, the Archangel of War, who is the second oldest created being in the universe and still able to kick the butt of anyone who gets in his way. With a battleaxe.
More grandpa-like is Barrin, Master Wizard, in Urza's Saga and the following sets of the Weatherlight Saga. Thanks to some slow-time water, he stops aging and starts getting younger instead, so that he actually looks stronger, and, one might say, sexier with age, so that in the end he doesn't look a grandpa any more.
Don't forget Urza, who fought a war against his brother (the end of which destroyed most of a continent and started a world-wide Ice Age) then struggled aginst the plane of Phyrexia for several thousand years before personally suiting up (along with eight of his closest "friends") to take the fight directly to Phyrexia.
In Shadowrun, FastJack is one of the best Deckers/Hackers on the Matrix (both the old and the new.) He was born in 1999, which means that in 2nd edition he's close to sixty, and in 4th edition he's in his early seventies and perfectly capable of booting kids off of his (virtual) lawn.
In Pathfinder, there is a mythic path ability that can be taken for any mythic path that causes a character to not suffer the negative aging problems, but gain all the positives.