this teenage girl for some reason worked at a fashion magazine/design company/(something to do with clothes) – she apparently was really good at her job, and there was this other female (and male?) colleague that were jealous.
I think the teenager was hosting a party or something, and the jealous colleague(s) figured out her real age and exposed her
I had the impression that it’s a very light-hearted movie (if you didn’t think so already based on what I described), and very likely to be before 2000. The style of clothing.. or the period that movie is based on, reminds me of the Back To The Future series actually….