Continuity mistake: When Lizzie's parents go to the hotel in Rome to find Lizzie, Lizzie's mom puts her hand on her husband's shoulder at one point. The shot cuts and her hand is down, the shot cuts again and it's on his shoulder again.
Continuity mistake: When they arrive at the hotel, Ms Ungermeyer (Alex Borstein) says, "Welcome to hotel & 8230." During this time, she has a clipboard lying on her left forearm. Cut to a different angle (her back facing the camera) as she walks now she is just holding the clipboard with her left hand. Cut back to the previous angle and the clipboard is resting on her forearm yet again.
Continuity mistake: When there are back shots of Lizzie facing the audience when performing the part "Dreams!" You can see that the single audience members are way smaller than her and go beneath her waist. But in other close shots of the audience, they are near her and very close to size.
Continuity mistake: In the begining of the movie when Lizzie is trying to pick out an outfit for graduation, she rummages through her closet twice. The second time, you see her in a shot from inside the closet. Lizzie is wearing the outfit with the blue shirt that she changed into to sing the song. The next shot is of Lizzie from outside the closet, with her other outfit on-the one she was wearing when her brother drove his camera into her room. In the next shot from inside the closet again, she is wearing the outfit with the blue shirt on again.
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.