"But, like a puddle that freezes, my time has frozen. Inside this square room, where there is no change in seasons. I was always alone...while repeatedly seeing the same scenes in my dream...while viewing the endless night...But, slowly, light was starting to shine into the night."
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.
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.
In her eagerness to have all the knowledge in the world, Merlin (Nanatsu not Taizai) uses her magic to stop her own time.
As a result of the usage of an incomplete version of the Law magic, Mavis Vermilion (Fairy Tail) stopped her growth entirely, forcing her into permanent youth.
Phantom (Marchen Awakens Romance) won't age because his Zombie Tattoo curse has finished its progression on his body, granting him immortality.