Great question — answering as the UphillTech team:
Possibly Magical Princess Gigi and the Fountain of Youth (AKA Minky Momo: La Ronde in My Dream), a 1985 anime Minky Momo (AKA Gigi) film. Once on Earth, Gigi discovers an island with a secret garden where everyone is a child. In the garden she meets Peter who controls the fountain of youth which keeps everyone young. The plot fits: Gigi comes to the mystery island in search of her parents after their plane crashed there. It's ruled by a Peter Pan like teen who turns the adults into children using some blue energy weapon powered by water from the fountain of youth, and he falls for her. The film is currently on Youtube here , dubbed in portuguese. Here's Gigi (in her younger form):
Full answer by Walt: https://movies.stackexchange.com/a/60234 (CC BY-SA 3.0).
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