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.
Rumplestiltskin (Once Upon a Time) can only be killed with his Dagger or while being in a Enviroment without Magic
John Amsterdam (New Amsterdam) becomes mortal when he finds his One True Love
Lorien (Babylon 5)
Witches (Witches of East End)
Slender Man (Creepy Pasta)
Viking Deities (Norse Mythology)
Peter Pan (Peter Pan)
Sorcerers (Skulduggery Pleasant)
The Young Guardians (The Young Guardians)
Hunters of Artemis (Percy Jackson Books)
Anodites (Ben 10)
Ectonurites (Ben 10)
Ghost Freak (Ben 10)
Verdona Tennyson (Ben 10)
Stitch (Lilo & Stitch)
All experiments (Lilo & Stitch)
Skips (Regular Show)
Princess Bubblegum (Adventure Time)
Vera Moray/Granny Rags (Dishonored); via enchanted cameo
The armor Eternity grants its wearer, Archie, eternal life as the inside has its own timezone.
Shadow is engineered to be ageless, and is over 50 years old despite his youthful form.
The Four Founders of Eden are all Dignified Power users, and thus mastered their life force to the point of ceasing their aging.
Kouji, like the Four Founders, is a Dignified Power user, and has ceased aging for a long time.
Prime Minister Fujiwara, despite being a normal human, has still mastered his life force like the Dignified Power users, and stopped aging.
Younger Toguro and his older brother wished to become demons, preventing them from aging.
Elder Toguro and his younger brother wished to become demons, preventing them from aging.
Adam Monroe never aged because of his regeneration.
Jacob was at the same age for over 2000 years.
Androids such as 18 (DBZ) are immune to aging since they're made of metal.
Jumba created artificial genetic experiments, such as Stitch, and as such he created them that don't age.
Jumba's experiments, such as Angel, are immune to aging.
Mavis is a 118-year-old vampire.
Because he was released from his pod early, Superboy (Young Justice) will never physically age.
Hidan (Naruto) is the successful product of the Jashin religion's experiments of immortality, and cannot die of injuries, but can die of hunger and natural causes; in essence, he's the inverse of a typical semi-immortal.
Sesshomaru (InuYasha) is a full-blooded demon who doesn't suffer from old age as he seems to be over 500 years old with the appearance of a 19 year old.
InuYasha bears demon blood within him, and has lived for about 200 years and looks like a 15 year old.
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.