The Captain Commander Shigekuni Yamamoto-Genryuusai is the leader of the Shinigami and by far the oldest, and he's been around so long and been part of so many battles that he's nearly impossible to beat in a one-on-one fight. He's more heavily scarred than any of the other shinigami, and tends to refer to other characters as "brats", and won't hesitate to belittle them if he thinks they aren't living up to their role as protectors of the spirit world.
Second Espada Baraggan Louisenbairn is the physically oldest-looking and one of the most powerful members of Aizen's Arrancar army. When Aizen and his cohorts are trapped by Yamamoto's fire during the assault on the fake Karakura Town, Baraggan takes command without any objects from his fellows (including the First Espada). It also helps that before Aizen came along and usurped him, he was the legendary God-King of Hueco Mundo and Hollowkind, and the light novels would reveal he's actually one of the oldest living Hollows in existence.
Sternritter "N" Robert Accutrone is one of the visibly oldest of the Quincy serving Yhwach and tough enough to give Shunsui a hard fight while leaving lasting injuries. It's even implied he was part of Yhwach's original Quincy army, given he knows better than most how absolutely little Yhwach cares for his own.
Fairy Tail: Among Zeref's Spriggan 12 who are his elite forces, August is the only elderly among them (at least visibly), but also the most powerful. His age is around the same as Makarov's, only slightly younger by a year or so, although he is much younger than Zeref himself. He is also Zeref's secret child.
Gintama has several, most of which are veterans from the Joui War.
First, there's "Mademoiselle" Saigou. These days he's a Drag Queen operating an okama bar and trying to raise his son by himself after the death of his wife, but back in the day he was a fearsome One-Man Army known to charge headfirst into battle wearing nothing but his white loincloth, which would then be dyed red by the blood of his enemies as he beat them to a pulp.
Then there's Jirocho, who's currently boss of his own yakuza family and one of the Kabuki District's Four Devas along with Saigou. In his youth he was a hooligan and a troublemaker to constantly picked fights and eventually got into organized crime, before signing up for the war when the Amanto came to Earth. As a testament of his power, he's capable of crossing swords with the likes of Gintoki even in his old age.
Housen of the Yato clan was once the captain of the 7th Division of the Harusame, the largest crime syndicate in the universe, and was strong enough to fight Umibozu to a standstill in their prime. These days he's retired himself to Yoshiwara, which he rules with an iron fist, and it takes the combined efforts of the Yorozuya and half of the Hyakka just to stay alive long enough to kill him.
While they are not even past their thirties, Gintoki, Katsura, Takasugi and Sakamoto are all veterans from the last generation to participate in the Joui War. Their might as warriors was unmatched, leading to them being dubbed the "Joui Four". These days they have gone their separate ways, but still remain a force to be reckoned with.
Lehm from Jormungand. He is a former Delta Force captain who served in the Gulf War and has years of combat experience (with scars to show for it). He is one of the senior members of Koko Hekmatyar's cadre of bodyguards and often serves as a leader and mentor to the team. Lehm is skilled with many firearms and even acts as a sniper when needed.
Maximilian Jenius is this by the time of Macross Delta: Absolute Live, having gone on to serve as a Private Military Contractor since the events of Macross 7. Even 68 years after Space War One, he has not lost his touch as the best Ace Pilot in the franchise, as he is able to single-handedly give the entirety of Delta Squadron a run for their money during training with his Ace Custom YF-29 Durandal and gets to fight alongside his own granddaughter Mirage in the finale.
Grey Stoke in both Mobile Suit Crossbone Gundam and later on V Gundam Gaiden. Given that he is Judau Ashta and these events occur almost 50 years after his previous adventures, it's understandable that he is among the most experienced soldiers around at the time both stories take place.
Signum in Lyrical Nanoha becomes this after being transported to the modern Mid-Childan society, where her old Belkan sensibilities undergo a transformation to adapt to a society based on (non-)humanism rather than wartime duty and knightly honor. Unlike Vita, who finds herself in training the new generation of mages in this new world, Signum continues doing what she did her entire, very long life—fighting—and declining opportunities to do anything else. While not as grumpy as the typical examples of this trope, she does call out the rookies (e.g. Teana) on their mistakes.
Naruto offers Hiruzen Sarutobi and Onoki, the Third Hokage and Tsuchikage respectively. Both were students of the founding leader of their respective villages and have lived through three Shinobi World Wars. Despite their power and skill both are also notably past their prime, Hiruzen's chakra pool having dwindled due to age and Onoki suffering literally debilitating back pains. Danzo also qualifies to an extent, but has notably taken to avoiding direct battle in his old age due to his fear of death.
Most significant is Sergeant Rock Shirakabe, who struggles with being saddled with the younger hunters from Drankam whom he must protect and train. Being so used to this makes him shocked to find out that Akira is a Humble Hero who lightly admires him. Shirakabe makes a Give Me a Reason speech to Katsuya and becomes a Sink or Swim Mentor to New Meat Togami.
Thorkell "The Tall" from Vinland Saga is fifty during his first appearance in the manga, and has recently been forcibly retired from the Jomsvikings due to advancing age. Despite this he keeps showing up to battles because he likes fighting, and is still a complete terror who is Famed In-Story for his long life as a career warrior.
