The Little Mermaid (1992)

S1E12 Red

Following Ariel's party, King Triton begins to feel his age weighing on him. His luminous glowfish decides to help by giving him a magical age stone that transforms Triton into a young boy named Red. When Ariel and Sebastian uncover the truth behind Triton's transformation, they set off on a quest to track down the jewel's source, discovering that it leads to a powerful sorceress who can only be defeated by the true King Triton.