I believe Sara is 11 as of her summer outfit. Back then, they may have been flirting with the idea of actually aging the characters over the years. She's pretty exceptional as far as developing as an actual child would as far as GBF is concerned.
Basically, when introduced to her in her first story, Sara is 9. A couple years later, she's 11. Everyone else in the story, especially characters who are part of the main story-line? Still the same age.