Other mistake: In the scene where Paolo leaves the stage after everyone discovers that he can't sing, his bodyguard Serge stops him and Paolo suddenly loses his Italian accent. Paolo sounds very American when he says "You work for me, Serge."
Other mistake: At the graduation, when the girl says to Kate, "It's like watching those poor animals on the discovery channel," you can see Kate mouthing her lines.
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.