He even volunteers to rejoin the war effort as an adviser to Commander Karrde when they ship out to Egypt.
Aburame Katsu from The Middle Ground (A Naruto fanfiction) is so bad-ass he can make Jounin shake in their boots by drinking tea. His son-in-law Aburame Shibi considers him to be scarier than his maternal grandfather who was a prison torturer. In fact he's so bad-ass he can take down a minor in-clan rebellion by just making it generally felt he disapproves of it, without once even mentioning it. That's how bad-ass he is.
The Firefly fic Forward has Shepherd Book further cement his badassery multiple times over the story, including having a scene where he silently knocks out several mercenaries while Mal keeps their leader distracted, and a particularly memorable moment where he saves River's life by facing off against an entire horde of Reavers singlehandedly, armed with nothing but a pistol and a sword.
Fuhrer Grumman, of Fullmetal Alchemist, becomes this in the third part of the Elemental Chess Trilogy. He spends the first part being a Chessmaster who's pretending to be a Cloudcuckoolander and having great fun at everyone else's expense, and is absent for most of the second part, but in the third installment he survives an assassination attempt, utilizes a secret escape passage to fool everyone into thinking he didn't survive it, and then comes Back from the Dead to rescue his grandson-in-law from execution by firing squad.
The Arbiter in The Last Spartan is in his golden years by the time he and Master Chief meet again, but even then, charges headlong into a warehouse full of mercenaries holding the Spartan prisoner with only Garrus Vakarian for assistance.
In the Uplifted series, Joachim Hoch becomes one in the first interlude. In his late 80's, having battled multiple bouts of cancer and survived World War II, he is still gearing up to lead the push to take back Rannoch from the Geth. He has decided that he can't even think of dying until he has set foot on, and preferably liberated the Quarian homeworld.
Harry's martial arts instructor in Harry Potter and the Gift of Memories was over seventy and described as capable of routinely beating Harry "six ways to Sunday with one hand tied behind his back, hopping on one foot, while blindfolded."
In the Medieval II Total War fanfic/Let's PlayA Scotsman In Egypt, King Edward of Scotland faces the Mongol Prince Subatai The Merciless in single combat. At the time of the duel, Subatai is 26 years old, and Edward is pushing 70. Place your bets on who wins, ladies and gentlemen. Hint: Don't choose Subatai.
Legendary Genesis's Headmaster Arcanine is remarkably strong and fast, and his senility makes him even more dangerous. Thinking his academy is under attack, he nearly ends up killing a couple of soon-to-be students.
Captain Steel Song of the Dusk Guard is pushing 60, races the morning sun each day (and wins), and can kick manticore tail without much trouble.