Yvonne - I believe you are thinking of "Hypergirl" written by Crystal Wimmer and part of the Season 6 fanfiction. It's the one where Mxytsptlk (did I spell that right? Even close?) uses magic on Lois & Clark's daughter Laura to speed up her growth rate. She grows up and turns into a super-powered rebellious teenager within a few days.
Smile and the world smiles with you ... frown and you're just giving yourself wrinkles.
