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.
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, King Piccolo (Dragon Ball) wields frightening power, easily outmatching 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) is a very strong swordsman despite his body often feels the effects of old age.
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.
Despite being 91-years-old, Kakuzu (Naruto Shippuden) is one of the most powerful and dangerous member of the Akatsuki.
Despite his old age, Whitebeard (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 years old pirate and a master swordsman.
Marine Admirals (One Piece)
While in his late 50s, Kizaru (One Piece) is an extremely powerful Marine Admiral.
Despite being a blind elderly man, Fujitora (One Piece) is an extremely powerful Marine Admiral.
Despite being in his 70, Monkey D. Garp (One Piece) is still considered one of the strongest marines alive.
Kureha (One Piece) is a genius doctor who has lived up to the age of 141.
While over 70 years old, Silvers Rayleigh (One Piece) is still one of the strongest people in the world.
Despite being nearly 80 years old, Sengoku (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.
All Minks (One Piece) are natural born fighter who retain their physical prowess even in old age.
Though elderly, Nekomamushi and Inuarashi (One Piece) are the strongest of the Mink tribe.
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 (One-Punch Man) is ranked 3 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 his age, Joseph (JoJo's Bizarre Adventures) is still a capable fighter, able to hold his own against most Stand Users.