Factual error: In the scene where Ms. Ungermeyer returns to the hotel and takes the elevator upstairs, Sergei detains her in the elevator (to give Lizzie time to get into her room) while the classmates, including Kate, walk up the stairs. Since the rooms were on the first floor, Kate should have reached her and Lizzie's room before Ms. Ungermeyer; however, they enter the room at the same time.
Factual error: When Lizzie was performing, there's no stairs in the shots of the crowd.
Factual error: In the scene where Paolo first meets Lizzie at the Trevi fountain, wouldn't he be swarmed by an army of photographers or at least a bodyguard or two? A popstar as famous as him probably couldn't go out alone in public like that.
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.