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.

meanwhile Amira is technically 5 years old

Even new years lines from zodiacs say how many years you spend traveling with them.

Wilfriback said:

Even new years lines from zodiacs say how many years you spend traveling with them.

And they don't age.