Perilous Old Fool

"Ole fools, brave, perilous ole fools, why did ye try it?"

—Skipper, from Triss, mourning Bluddbeak and Ovus

He's an Old Soldier, all right — a very old soldier who is adamant he's Made of Iron and good for another battle, even though it's woefully clear that said iron is showing a lot of wear and tear and his fighting days are behind him.

To fit this trope, the subject must be far too old and feeble to do battle anymore, or perhaps seriously disabled (either from previous battles or illness), or both. The subject must also be in denial and insist they're good for another round. Remember: Perilous Old Fool.

Quite often, did have a well-earned Badass reputation way back when, but has suffered some very literal Badass Decay. Very often ends up with a bad case of dead, and Curb Stomp Battles are pretty much the norm.

If the soldier is willing to sacrifice himself because he's old and infirm and the others have their lives ahead of them, or knows he's too old for a fight but wants to go out in a blaze of glory, that's Death Seeker, not this trope. If someone's old as dirt but still has the chops, that's Badass Grandpa (or Never Mess with Granny if it's a woman) and should only be listed here as a subversion if they're believed to be this.

Examples of Perilous Old Fool include:

Comic Books

One Archie comic showed several veterans chasing a thief—but they no longer had their youthful stamina. The thief even said "This is where I lose the geezer brigade".

Film

Independence Day - Russel hasn't been in a fighter plane since the Vietnam War, but he wants payback for having been (supposedly) abducted by the invading aliens. He contributes to the battle by making a Heroic Sacrifice just before the giant alien ship can use its Wave Motion Gun on Area 51.

Literature

Bluddbeak, the redkite from Triss, is one of the Trope Namers. He has rheumatism. He's blind. And he's adamant that he's still the great adder-killer he once was and can take on a trio of the snakes. Oh, is he wrong. The other "fool" Skipper is referring to in the quote is Ovus the owl, who feels the need to help Bluddbeak out despite his own great age and rheumatism. He fares better only in that he gets a decent burial, while all that's left of Bluddbeak are scattered feathers.
The David Gemmell book Legend is a major subversion of this. By all standards of the setting Druss is way too old to fight another war and even Death itself mocks him about it. He has crippling arthritis in his hands and his body is wrecked by decades of fighting. However, he quickly proves that he is still the deadliest man alive and still strong enough to kill hundreds of enemy soldiers in hand-to-hand combat.
In one Ciaphas Cain novel, a retired Imperial Guard veteran joins the militia to help fend off a Chaos invasion, and although he's very enthusiastic it's clear he's not quite all there.
Defied in By The Sword when a mercenary is left behind because he'd be tempted to take part in "one last battle", despite being very old and having only one arm.
Ajax the Archer in Myth Conceptions.
Cohen the Barbarian of Discworld is edging close to this by his first appearance in The Light Fantastic, with his bad back and lack of teeth, but once he gets his back straightened out and a pair of (diamond!) dentures, he returns to being a full-time Badass.

Live-Action TV

Corporal Jones in Dad's Army, a veteran of wars going right back to 1886, who saw fighting in The Sudan (1886), South Africa (1899-1902), a bit of WW 1 (1914-1918)... and who in 1940, aged 80, wants to come back for his fourth.
Doctor Who: The Doctor thinks Brigadier Lethbridge-Stewart is one of these in "Battlefield", if his line over the Brig's unconscious (assumed dead) body is any indication.

Professional Wrestling

A common complaint against several venerable and famous wrestlers is that they're too old and physically damaged (either from their work or from drugs) to properly wrestle anymore but insist they're the stars they once were and thus hog the limelight from younger, fresher talent. Let's leave it at that, please.

Tabletop Games

In Dungeons and Dragons this is the typical way for giff to die. Their culture has no respect for the infirm, so aging giffs will usually stay in active military duty, trying to prove they still have it.

Video Games

Metal Gear Solid 4: Guns of the Patriots does the classic subversion - Snake keeps insisting he's able to fight, partly because he's a Death Seeker and partly because he is still able to fight out of sheer Determinator factor. However, he limps around in cutscenes, is missing half his face and has developed some form of lung disease from what had been badass smoking when he was younger.

Western Animation

Subverted in Mulan; her father is aware that he is not in the best condition to fight, but is willing to die for his country and honor. Naturally, Mulan doesn't find this acceptable.

Truth in Television

Several veterans of the Boer War wanted to fight in World War II, being turned away because they were too old. There was one case where the man in question dyed his hair — but the heat made the dye run, revealing his gray hair and thus his age.