Continuity mistake: Towards the end of the film, Mattie hands Jenna the dream house he made for her 13th birthday. There is hardly any "wishing dust" on the roof. However, when she goes outside and sits on her front porch, there is plenty of wishing dust all over the roof of the toy house now.

13 Going On 30 (2004)
Plot summary
Directed by: Gary Winick
Starring: Mark Ruffalo, Jennifer Garner, Judy Greer, Andy Serkis
Its Jennas birthday and her friend matt gives he a dream house with wishing dust on it. Afterwards the popular people and the guy shes got a crush on comes around and Matt goes to get his keyboard. Tom-tom says they gonna play 7th heaven and tells her to go to the closet and wait for the guy she fancys to go in there and do what ever he wants to her. They all leave and on the way out Matt enters and Tomtom says to Matt that Jenna is waiting in the closet for her. Matt goes in and Jenna is blindfolded and she removes her blindfold and sees its Matt and thinks Matt chased of all the popular people away and gets mad. She wishes that the was 30 years old and then the wish comes true because of the wishing dust. She finds out that she got everything she wanted, had plastic surgery is still friends with Tom tom and works for her fave magazine. She also figures out that her "friend" isnt a true one and the guy she fancies is really ugly and works as a cab driver.
Jenna: Wait, listen to me. I'm 13.
Lucy: Jenna, if you're gonna start lying about your age, I'd go with 27.
Trivia: When Jenna wakes up as 30, she is in the same apartment the 30-year-old woman in the magazine was in when she was 13.
Question: When Jenna has turned 30 and is going to the magazine party she has a dress on then asks her 13 year old neighbour if she can see her underwear to which the girl replies something like "I think that's the point." They then smile and laugh to themselves. Could anyone explain this joke?
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Answer: Jenna asked the girl, "Can you tell I'm wearing underwear? Because I totally am". This is because when she was 13 just a day before, she was wearing big underwear. Now she's wearing a thong, which wouldn't be visible when wearing a dress.