Despite his advanced age, Genryūsai Shigekuni Yamamoto (Bleach) was considered to be the strongest Soul Reaper in existence for the last millennium.
Ichibē Hyōsube (Bleach) is one of the oldest Soul Reapers, possibly older than even Yamamoto, and is also one of the strongest beings in existence.
Yhwach (Bleach) is the first and most powerful Quincy to have ever existed.
Though not as powerful as he was in his prime, even in old age, Demon King Piccolo (Dragon Ball) wielded frightening power, easily outmatching skilled martial arts like Son Goku and beating him to the point where his heart temporarily stopped.
Despite his advanced age, Master Muten Roshi (Dragon Ball series) is a very skilled and experienced warrior...
... enough to earn the respect of the God of Destruction.
King Bradley/Wrath (Fullmetal Alchemist) was a very powerful swordsman despite his body often feeling the effects of old age.
Umibozu (Gintama) may be past his youth, but he retains the tremendous combat prowess enough to be regarded as the strongest in the universe, that even Utsuro respects.
Master Son (Gintama) remained in the battlefield for over two centuries, due to his mastery of Qi Gong.
Walter C. Dornez (Hellsing)
Aged 120, Isaac Netero (Hunter × Hunter) can fight toe-to-toe with the strongest Chimera Ant Meruem.
Zeno Zoldyck (Hunter × Hunter) attempts to assassinate the Phantom Troupe's leader, Chrollo.
Hagoromo Ōtsutsuki, the Sage of Six Paths (Naruto)
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.
Despite being 91-years-old, Kakuzu (Naruto Shippuden) is one of the most powerful and dangerous member of the Akatsuki.
Aged 65, Mifune (Naruto) is the Samurai General of the Land of Iron and one of its greatest swordsman.
Despite his old age, Edward "Whitebeard" Newgate (One Piece) was still considered one of the most powerful men in the world before his death.
At the age of 68, Charlotte "Big Mom" Linlin (One Piece) is one of the four most powerful pirates in the world.
Brook (One Piece) is a veteran 90 year old pirate and a master Gentle Style swordsman.
Marine Admirals (One Piece)
While in his late 50s, Kizaru (One Piece) is an extremely powerful Marine Admiral.
At age 70, Lao G (One Piece) is a powerful grandmaster of the Earth Sage Fist and one of the strongest executives of the Donquixote Pirates.
Despite being a blind elderly man, Fujitora (One Piece) is an extremely powerful Marine Admiral.
Despite being in his 70s, Monkey D. Garp (One Piece) is still considered one of the strongest marines alive...
...defeating legendary figures like Don Chinjao, a 500,000,000 million bounty pirate with nothing but his fist.
Kureha (One Piece) is a genius doctor who has lived up to the age of 141.
While over 70 years old, "Dark King" Silvers Rayleigh (One Piece) is still one of the strongest people in the world.
Despite being nearly 80 years old, Sengoku "The Buddha" (One Piece) was a Fleet Admiral of the Marines and one of, if not the strongest amongst their ranks, able to even battle the Blackbeard Pirates by himself.
Though elderly, Nekomamushi and Inuarashi (One Piece) are the strongest of the Mink tribe.
All Minks (One Piece) are natural born fighter who retain their physical prowess even in old age.
Even at age 56, Ashura Doji (One Piece) is still one of the most powerful samurais of Wano.
Fukurojuku (One Piece) is the elderly captain of the Orochi Oniwanbanshu, the most elite squad of ninjas in the Wano Country.
Despite being elderly, Hayato Fūrinji (History's Strongest Disciple Kenichi) is a powerful martial artist in prime condition.
Cologne (Ranma ½) looks like a deformed she-gnome, measures her age in centuries rather then years, and can still beat down any two of the teenage martial artists at the same time.
Happosai (Ranma ½) one of the two by-far most skilled characters in the series, and likely easily the most powerful chi-user.
Though not as powerful as she was in her prime, Genkai (Yu Yu Hakusho) remains a formidable fighter.
Despite his old age, Bang/Silverfang (One-Punch Man) is ranked 3rd among the strongest of the Hero Association and founder of his own brand of martial arts.
In his life as a Human in his sixties, Uraltugo Noi Nueph (Magi The Labyrinth of Magic) is genius master magician and one of the three Magis of Alma Torran.
Even at age 69, Joseph Joestar (JoJo's Bizarre Adventures Part III Stardust Crusaders) is still a capable fighter, able to hold his own against most Stand Users.
Even at the elderly age of 87, Vasco Strada (Highschool DxD) is still one of the most powerful Exorcists of the Church.
One of the greatest military figures of King Sho's era, Ou Ki (Kingdom) was a Great General of Heavens who achieved victories that numbered in the tens of thousands against many formidable foes which earned him the nickname of the Monstrous Bird of Qin...
...and became a living legend in his mid 50s as the last surviving member of the Six Great Generals, the six most dangerous generals of the state of Qin.
A Great General of Heavens who displayed the physical prowess worthy of the title, Duke Hyou (Kingdom) was the State of Qin's greatest Instinctual-type warrior as well as the oldest.
The last surviving member of the previous generation of Zhao's Three Great Heavens, Ren Pa (Kingdom) is a man recognized by all of China as one of the greatest warriors of King Sho's era of warfare...
...and as a Great General of the Heavens who holds the power of 100 men.
Mou Gou, Haku Rou (Kingdom)
One of the most experienced commanders in the entire State of Qin, Chou Tou (Kingdom) was also the oldest generals in the Qin Army, having served in the military since the age of 15.
Ordo, the King of 50 Mountain Tribes (Kingdom) is one of the oldest and most powerful Great Generals of the state of Yan as he served alongside famous figures like Geki Shin, the Savior of the Yan Kingdom and the Military God, Gaku Ki.
The oldest and the last surviving member of the Wei Fire Dragons, Gai Mou the Raging Bull (Kingdom) is one of the State of Wei's strongest warriors,...
...having taken the heads of over a hundred enemy generals and thousands of their soldiers.
Having served in the northern Ganmon stronghold on the Zhao/Xiongnu border for over a decade, Ba Nan Ji, the Demon of Ganmon (Kingdom) is one of the State of Zhao's strongest warriors and serves as Ri Boku's right hand man and deputy.
The former deputy and Right Hand Man of Rin Shou Jo of Zhao's previous generation of the Three Great Heavens, Gyou' Un (Kingdom) represented over a decade of Zhao martial prowess and served as one of the oldest Instinctual-type warriors.
Known as the Left Hand and Shield of Rin Shou Jo, Chou Ga Ryuu (Kingdom) was one of the state of Zhao's most talented commanders as well as the oldest having served as one of best tacticians during King Sho's era of warfare.
Master Hades (Fairy Tail) is one of the oldest and most powerful wizards to have ever lived in Earthland.
August (Fairy Tail) is one of the oldest and most powerful member of the Spriggan 12.
While he's gotten old,Georg Carmine (How a Realist Hero Rebuilt the Kingdom) is still one of the strongest (if not the strongest) in Elfrieden Kingdom. Even opening his eyes is overwhelming.