Retired Badass Roundup

The world has changed. Years have passed since these old heroes, villains, or neutral characters all were in their prime. In fact, most of them are now Retired Badasses. But the world needs them, one last time, for one last battle. Or they're just getting old and need to feel alive again before they're too old to do anything anymore.

Compare to Putting the Band Back Together, when its any kind of group. Or to the Magnificent Seven Samurai, when its about any group of people who are Badass. What makes this different from those tropes is that this trope is about those who are old and have had their turn coming back.

For the singular version, look for Back in the Saddle. See also One Last Job.

Examples of Retired Badass Roundup include:

Anime and Manga

In Gun X Sword, there was that fan-favorite episode featuring the El Dorado Five - a bunch of former Hot Blooded Super Robot pilots, now octogenarians who do nothign but hang around the local bar chatting 'bout the glory-days. Untill suddenly, a Monsterof The Week appears, (as always happens when Van is around), only this time, the old coots decide to pull their Combining Mecha out of mothballs for one last fight... Van, out of respect for the pride of these old warriors, decides to sit out the fight, only providing a bit of timely aid from the sidelines. It was popular enough that the El Dorado team came back as the collective Big Guy for Van's Ragtag Bunch of Misfits in the second half of the series. And when Gun X Sword was finally featured in a Super Robot Wars game, the El Dorado V was a recruitable, permanent mecha for your team.
Dragon Ball: Buu Saga

Film

Pretty much the entire plot of Once Upon a Texas Train. A train robber is released after twenty years in prison and recruits his old gang to try and rob the same train that he was originally caught trying to rob. The man who caught him rounds up his band of retired Texas Rangers to go after him.
This is also the entire plot of the movie RED.
Star Trek VI: The Undiscovered Country: has all of them meeting for one last time. There was supposed to be a bunch of recruitment scenes of picking them all up from different places, but was cut for budget reasons.
Space Cowboys: a movie in which a Russian satellite is falling to Earth and so four retired astronauts that never got into space go up because no one else is old enough to remember how to fix it.
The Man in the Iron Mask: The three Musketeers meet to try and replace the king with his twin brother.

Literature

Happens at the end of the book Belisarius by Robert Graves (of I, Claudius fame), when the titular character has to defend Constantinople with literally no army. So he gathers his former soldiers, now old bureaucrats and retired commanders. And they still win.
Terry Pratchett's Discworld novels Interesting Times and The Last Hero feature Cohen The Barbarian and the Silver Horde. Their entire shtick is this trope.

Live-Action TV

Star Trek: Deep Space Nine: the episode "Blood Oath" where Kang, Kor, and Koloth, all Klingons from Star Trek meet for one last quest to fight against the murderer of children, the Albino.
Buck Rogers in the 25th Century had a squad of old retired fighters led by Peter Graves come back for One Last Job.
Within the 1980 version of Zorro, a bandit troupe threatens the town and Don Diego's father calls in the badass crew he used to work with. Their skills have noticeably degraded in the time they've been retired, so Zorro has to do their job behind the scenes without letting them realize their failure.

Mythology

The Argonauts, making this Trope Older Than Feudalism.

Video Games

Fallout: New Vegas: In the optional quest "For Auld Lang Syne", you recruit five retired Enclave soldiers to fight on your side. In the epilogue, it's stated that they go down in legend for how much ass they kick, reminding people just how scary the Enclave can be.

Western Animation

Extreme Ghostbusters: While Egon was leading the new team of Ghostbusters, the old gang came back for the series finale.
The Venture Brothers: In the episode "Past Tense", when both Brock and Dr. Venture are kidnapped, Hank and Dean recruit the original Team Venture to help rescue them. Hilarity Ensues.
The Powerpuff Girls parodies this in the episode "Fallen Arches": the leader of a legendary team of villains, the "Ministry of Pain" decides out of the blue to get his team back together and resume operations despite all of them having gotten really old. Blossom for whatever reason stops the other girls from doing something since they must "respect the elderly" so they trick the heroes who stopped them in the old days, Captain Righteous and his sidekick Lefty to fight them, despite them having had a falling out. The "battle" ends with all the elderly in intensive care and Blossom being chewed out by the news anchor about the fact that We Could Have Avoided All This if only they had intervened directly.
Avatar: The Last Airbender had the Order of the White Lotus and their battle to retake the Earth Capital of Ba Sing Se.
Spider-Man: The Animated Series had World War II superheroes come out of retirement to aid Captain America (comics) against a newly rissen Red Skull and his son Electro (alternate continuity) and their army of robots.