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.
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.
Master Roshi (Dragon Ball) eats constantly Paradise Grass, which prevents him from dying of old age.
Tomiko Asahina (From the New World) restores the length of her telomeres, allowing her to extend her life indefinitely.
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.
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.
China (Axis Powers Hetalia) is 4000+ years old, but appears much younger due to being the only country that is truly imortal.
The Golden Tyrant/Judas Iscariot (Seikon no Qwaser) has lived for over two thousand years since the time of Jesus Christ.
As one of the first generation of Angels, Michael (Highschool DxD) is over 10,000 years old , having lived since the time of the Biblical God.
As one of the first generation of Angels, Gabriel (Highschool DxD) has lived for over 10,000 years.
The Ancient Dragon (Seiken Tsukai no World Break) is the most ancient metaphysical of them all, as he has lived completely unchanged by the passage of time for countless ages.
Demi-goddess Rory Mercury (GATE) retains the appearance of a 12 years old girl, despite being 964 years old.