Though not as powerful as he was in his prime, even in old age, King Piccolo (Dragon Ball) wields frightening power, easily outmatching Goku and beating him to the point where his heart temporarily stopped.
Walter C. Dornez (Hellsing)
Tsunade (Naruto) appears to be a young woman, but she is already in her 50s, using her superhuman strength and a transformation technique to mask her physical appearance.
Anton Herzen (Professor Layton)
Iroh (Avatar:The Last Airbender)
Iroh (Avatar:The Last Airbender) the Dragon of the West.
The Avatars (Avatar:The Last Airbender/Legend of Korra) of the past.
Ezio Auditore da Firenze (Assassin's Creed)
The Batman of Earth-31 (DC Comics)
Master Yoda (Star Wars), considered to be one of the most skilled users of the Force in the universe.
Big Boss (Metal Gear), the greatest soldier of the 20th century.
Yamamoto (Bleach) is a fearsome fighter for his age.
King Bradley/Wrath (Fullmetal Alchemist manga/Brotherhood) is a very strong swordsman despite his body often feels the effects of old age.
Master Roshi (Dragon Ball)
Shepherd Book (Firefly/Serenity)
The End (Metal Gear Solid 3: Snake Eater), the father of modern sniping.
Rupert Giles (Buffy the Vampire Slayer)
Master Xehanort (Kingdom Hearts)
Splinter (Teenage Mutant Ninja Turtles)
General Alister Azimuth (Ratchet & Clank)
Albus Dumbledore (Harry Potter)
Hex (Ben 10), a master magician at age 62.
Salim al Kupar (Sly Cooper)
Mr. Miyagi (The Karate Kid)
Kakuzu is 91-years-old who uses his power to get new hearts from others, as he stated remembering the fight with the First Hokage.
Despite his age, Whitebeard (One Piece) was still considered one of the most powerful men in the world before his death.
Despite his age, Garp (One Piece) is still considered one of the strongest marines alive.
Octavio (Sly Cooper) may be old, but he's incredibly fast and strong.
Deathstroke/Slade Wilson (DC Comics). Despite being around 60 years old, he is considered the greatest mercenaries of all time due to a failed medical experiment he had volunteered for when he joined the military.
Master Zik (Sonic Lost World), despite being over a thousand years old and in semi-retirement, is able to fight with the strength and agility of someone a fraction of his age.
Despite being over 100 years old, Cap'n Cuttlefish (Splatoon) was able to serve competently as Agent 3's handler.
DJ Octavio (Splatoon) is a challenging foe despite being over 100 years old.
Rowen Ilbert (Tales of Xillia)
Heihachi Mishima (Tekken)
Though not as powerful as she was in her prime, Genkai (Yu Yu Hakusho) remains a formidable fighter.
Despite his advanced age, Master Roshi (Dragon Ball) is in good condition, having once been hailed as the strongest man in the world.
Despite a ninety year old elderly man (Marvel Cinematic Universe)...
...Stick is a extremely strong martial art master, with physical abilities completely untarnished by old age.
Madame Gao (Marvel Cinematic Universe) is a four hundred year old centenarian master strategist and a martial art master.