Users of this ability never age, and as a result, they stay young forever or at least never suffer the ravages of aging. Because of this, the user will always be at their physical prime. The user will also be immune to aging abilities, such as Age Acceleration or Reversal.
User is still vulnerable to everything else that would affect other beings, from disease to mortal wounds.
Users may stop aging only after they reach a certain point.
Depending on what that point is, user may never reach adulthood.
A number of organisms experience "biological immortality", which is essentially the real-world version of this power. Although they don't live forever, these organisms remain in their physical prime for the duration of their lifespan and do not "grow old" in the same way that humans do.
Unlike other forms of Immortality, this version is really hard to detect whenever it comes to physiological science. To basically put it, users of this power are still mortal, but except their bodies don't age at all, making this form of immortality half-baked in terms of functionality.
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.
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.
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.
Timmy Turner (The Fairly OddParents) made a secret wish that everyone on Earth would stop aging so he would stay 10 years old and could keep his fairies forever, though 50 years later the Fairy Council undid the wish when they discovered it.
Jägergenerals (Girl Genius) are the eldest and possibly the original Jägermonsters, making them 600-700 years old.
Jumba (Lilo and Stitch) created artificial genetic experiments, such as Stitch, and as such he created them so that don't age.
Immortus (Marvel Comics) ceased his own aging process by his far-flung futuristic technology.
Benjamin Grimm/The Thing (Marvel Comics)
As a side effect of Aku's Time Travel spell, Jack (Samurai Jack) was rendered ageless; he has wandered the Earth for 50 years and hasn't aged a day.
Gems (Steven Universe) are hatched from the ground as fully formed adults and do not age further, being able to live indefinitely.
Because he was released from his pod early, Superboy (Young Justice) will never physically age.
Jenny Sparks (Wildstorm/DC Comics) was born "with the century" - on the stroke of midnight on New Year's Eve of 1900. As she grew she displayed certain superhuman abilities and skills, as well as immortality (not aging beyond her 20's).
China (Axis Powers Hetalia) is 4000+ years old, but appears much younger due to being the only country that is truly immortal.
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.
Road Kamelot (D.Gray-man) hasn't aged in 35 years, keeping her preteen appearance forever.
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 Z) 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 series) eats constantly Paradise Grass, which prevents him from dying of old age.
Majin Buu (Dragon Ball series) has existed since the dawn of time itself, and has shown no signs of aging since.
Mavis Vermilion (Fairy Tail) hasn't aged since casting an incomplete Law spell to save her friend Yuri Dreyar life, looking like 13 years old despite being roughly 118.
Tomiko Asahina (From the New World) restores the length of her telomeres, allowing her to extend her life indefinitely.
Demi-goddess Rory Mercury (GATE) retains the appearance of a 12 years old girl, despite being 964 years old.
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.
Muromi (Muromi-san) is completely ageless, having been alive since Pangaea over 300 million years ago.
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 Statue of the Outer Path, 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.
The power of the Hobi Hobi no Mi has given Sugar (One Piece) eternal youth. Despite her appearance, she is actually 22 years old.
Dr. Genus, (One-Punch Man) a centenarian, after rejuvenation.
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.
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.
The Golden Tyrant/Judas Iscariot (Seikon no Qwaser) has lived for over two thousand years since the time of Jesus Christ.
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.
Azuza Aizawa (While Killing Slimes for 300 Years, I Became the MAX Level Unknowingly) looks 17 years old, but has been "The Witch of the Highlands" for 300 years.