It's not Amy grew up, it's just that they changed her age. She's not the only character it has happened to within the games (or other Sonic universes, such as Archie's comic). The Chaotix when they were re-introduced in Sonic Heroes had their ages changed from their debut in Knuckles Chaotix as well. -True Red
OK, a while back, I read on your website that Amy's official age used to be eight, but after Sonic Adventure it was changed to twelve. How come she grew up, but none of the other characters in the Sonic world have aged a bit? I myself am emotionally hung up by this.