How to make childish clothing look more mature?

I'm a really girly girl (ruffles, glitter, etc) but I used to be a tomboy. I normally dress to kill, but I'm really just a geek at heart. I've recently come across collections from my tomboyish childhood stage: Pok�mon, Power Rangers, Super Mario Brothers, Teenage Mutant Ninja Turtles... the whole lot. I was wondering how I could pull off wearing childish things (T-Shirts, Backpacks, Belts) but still look mature. Is it impossible?