The user can negate the age of themselves and others/objects, thus causing a matter, atom, cellular stand-still and can no longer change and grow older or be changed younger, thus the targets are stuck at that age forever.
May need to do certain things (meditation) or consume certain substances (blood, tanna leafs) to stay un-aging.
Dracula (Bram Stoker's Dracula)
Live Action TV
Adam Monroe (Heroes)
Clan Tralesta (Star Trek: TNG)
Welsper (Ah! My Goddess)
Asuka Shikinami Langley (Rebuild of Evangelion)
Dark Zagi (Ultraman Nexus)
The Doctor (Hellsing)
Flame Hazes (Shakugan no Shana)
Gen/Zen Fudo (Aquarion Evol)
Kaworu Nagisa (Rebuild of Evangelion)
Lord Genome (Gurren Lagann)
Mari Makinami Illustrious (Rebuild of Evangelion)
Mavis Vermillion (Fairy Tail)
Quinella (Sword Art Online)
Rei Ayanami (Rebuild of Evangelion)
Shen Long (Dragon Ball)
Shinji Ikari (Rebuild of Evangelion)
Sugar (One Piece)
Ichigo Morino (Onegai Teacher); via Standstill
Kei Kusanagi (Onegai Teacher); via Standstill
Tenzen Yakushiji (Basilisk)
Tosuka (Highschool DxD)
Trafalgar D. Water Law (One Piece)
Zook (Martin Mystery)
Gumball Watterson (The Amazing World of Gumball)
Darwin Watterson (The Amazing World of Gumball)
Nyx (Fire Emblem Fates)
Cosmic Forge (Rifts)
Vampires (World of Darkness)
Elixir of Life (Harry Potter)
Cruxis Crystals (Tales of Symphonia)
Keromet (Sgt. Frog/Keroro Gunso)
Tenzen Yakushiji (Basilisk) having his symbiote "eat" away wounds and the ravages of time, maintaining his youth for over 200 years.
Shen Long (Dragon Ball) can grant eternal youth to the wisher, as Lord Slug and King Piccolo wished for their youth to be restored.
Due to a curse she placed on herself, Nyx (Fire Emblem Fates) has completely negated her ability to age.
By repeatedly regenerating her telomeres, Tomiko Asahina (From the New World) has largely negated her aging, maintaining her youth for 267 years.
Adam Monroe (Heroes) has regenerated so many times that his cells now regenerate and die in equilibrium, negating his aging.
Trafalgar D. Water Law (One Piece) can use the Perennial Youth Operation of his Ope Ope no Mi to grant eternal youth to another person, though he must forfeit his life in return.
Sugar (One Piece) ate the Hobi Hobi no Mi, which ceases her aging, and anyone who she changes into toys will also cease to age so long as they remain as toys.
Mitsuhiko Ishibori is the alias of Dark Zagi (Ultraman Nexus) when in possession of Hajime Yamaoka. After 18 years of possession, the host was actually old enough to be a middle aged man but his skin didn't age thanks to Zagi.