Professor Paradox (Ben 10) was displaced outside of time due to an "experiment gone wrong", rendering him biologically immortal, never aging nor needing sustenance and sleep.
Washio (Buso Renkin) is a homunculus, which makes him an ageless life-form, with immunity from diseases, making him very hard to kill.
Papillon (Buso Renkin) is a homunculus, which stops him from aging, and even prevents him from dying from his mortal disease, making him very hard to kill.
Kinjo (Buso Renkin) is a homunculus, which stops him from aging and immune to any mortal diseases and very hard to kill.
The Four Founders of Eden (Code:Breaker) are all Dignified Power users, and thus mastered their life force to the point of ceasing their aging.
Kouji (Code:Breaker) is a Dignified Power user, and has ceased aging for a long time.
Prime Minister Fujiwara (Code:Breaker) has mastered his life force like the Dignified Power users, and stopped aging.
José (Cybersix) was engineered by his father to have eternal youth, so even though he is chronologically an adult, he remains biologically a child, to prevent rebellion.
Shinigami (Death Note) will remain eternal, so long as they continuously use their Death Notes to extend their own lifespan when necessary.
Artificial Humans (Dragon Ball) such as 17 and 18 ceased to age since they are altered at a cellular level, while 16 is synthetic from the start.
Artificial Human 19 (Dragon Ball) is a synthetic creation of Dr. Gero who will not age.
Dr. Gero (Dragon Ball) converted himself into an Artificial Human, thus escaping old age for the sake of eternal life.
Due to a powerful curse she accidentally placed on herself, Nyx (Fire Emblem Fates) will never age.
Tomiko Asahina (From the New World) restores the length of her telomeres, allowing her to extend her life indefinitely.
Adam Monroe (Heroes) never aged because of his regeneration.
Mavis (Hotel Transylvania) is a 118-year-old vampire.
Pit (Kid Icarus) is an angel with Semi-Immortality, it was proven when Pit mentions that Medusa was already hard enough to deal with 25 years ago.
Jumba (Lilo and Stitch) created artificial genetic experiments, such as Stitch, and as such he created them so that don't age.
Jacob (Lost) was at the same age for over 2000 years as a result of becoming the guardian of the heart of the island.
After becoming the smoke monster, The Man in Black (Lost) became ageless.
Immortus (Marvel Comics) ceased his own aging process by his far-flung futuristic technology.
Kakuzu (Naruto) tears still-beating hearts out of his victims and integrates them into his own body, extending his lifespan so long as he continues this process when necessary.
Hidan (Naruto) is the successful product of the Jashin religion's experiments of immortality, and cannot die of injuries, but can die of hunger ; in essence, he's the inverse of a typical semi-immortal.
Sasori (Naruto) converted himself into a puppet, escaping old age and sustenance intake necessity; the only way to kill him is to attack his core of living flesh.
Madara Uchiha (Naruto) linked himself to the Gedo Mazo, extending his lifespan indefinitely so long as he remains hooked up to this life support. However, this did not stop him from aging.
Zetsu (Naruto) are ageless, as Black Zetsu is an artificial human created from Kaguya's materialized will, while White Zetsu are mutated humans.
Brook (One Piece) possesses eternal youth since his second life is supported by his Devil Fruit ability, and his living cell tissues have already rotted off before he came back to life.
Archie (Pokemon Adventures) wearing the armor Eternity, which grants him eternal life as the inside has its own timezone.
Kurousagi (Problem Children are Coming from Another World, Aren't They?) is no longer aging and has lived for over 200 years and still has the appearance of a 18 years old.
Shadow the Hedgehog (Sonic the Hedgehog) is engineered to be ageless, and is over 50 years old despite his youthful form.
Peter Pan (Valkyrie Crusade) doesn't age.
Anomalocaris (Valkyrie Crusade) is completely ageless, having been alive since the cambrian period over 500 million years ago.
Because he was released from his pod early, Superboy (Young Justice) will never physically age.
Younger Toguro (Yu Yu Hakusho) and his older brother wished to become demons, preventing them from aging.
Elder Toguro (Yu Yu Hakusho) and his younger brother wished to become demons, preventing them from aging.
As the incarnation of the natural world, Lala-Ru (Now and Then, Here and There) hasn't aged in over 600,000 years.
Muromi (Muromi-san) is completely ageless, having been alive since Pangaea over 300 million years ago.
Judd (Splatoon) has (somehow) been alive outside of cryogenic suspension for at least 2,000 years without aging.
Due to living in Neverland, Peter Pan (Peter Pan) never ages beyond that of a 12 year old
The Hero's Dog (Fable II/III) seemingly never ages.
Gems (Steven Universe) are hatched from the ground as fully formed adults and do not age further, being able to live indefinitely.
Timmy Turner (The Fairly OddParents) made a secret wish to stay everyone on Earth would stop aging and that he would stay 10 years old so he could keep his fairies forever, though 50 years later the Fairy Council undid the wish when they discovered it.
Being toons, Yakko, Wakko and Dot (Animaniacs) are all essentially immortal and ageless. They had a prominent career in the 1930s until they were locked in the Warner Bros watertower, resurfacing in the 1990s without aging a single day.
China (Axis Powers Hetalia) is 4000+ years old, but appears much younger due to being the only country that is truly imortal.