Revealing mistake: Lizzie has black nail polish and the same size of nails throughout the entire movie. Supposedly, Lizzie and Isabella are different people. But, when Isabella comes back, you can see she has the same color nail polish and the same size of nails as Lizzie. I don't think it's just a coincidence.
Revealing mistake: Right at the end of the movie, when Lizzie is singing on stage in front of thousands of people, it goes to a shot behind her and you can easily see that she's not actually there.
Revealing mistake: In the scene where Paolo and Lizzie are at the designer's studio, and Lizzie is talking to Paolo while wearing the green outfit, you can see the cord at the back of her neck that made the skirt pop up when she pulled the dangley things in front of her face. (00:43:45)
Answer: In the TV show, they are 13-14 years old. Assuming this is after the TV show, and we never see Lizzie, Gordo or Miranda celebrating their 14th birthdays (we see Kate celebrating her '14th' birthday and Gordo celebrating his Bar Mitzfah a year late), Lizzie and Miranda are 13-14, Gordo is 14 and Kate is 15 (it is mentioned in an earlier episode that she was held back a year in Kindergarten.) This is only an assumption, due to the TV show.