There have been many instances of this. Many of the girls in the David Hamilton films and the the Schulmadchen series were in their twenties. Also, like loverboy has pointed out, this is a good way for controversial films to actually get made and released.
My first look at Kate Beckinsale was in a movie called Anna Lee - Headcase (1993) http://www.imdb.com/title/tt0463866/ in which she played a sexually abused 14 year old. Stunning, even though she was about 20 at the time.
Francesca Neri was 36 when Edades de Lulú, Las (1990) http://www.imdb.com/title/tt0099484/ was released. I loved her portrayal of an innocent teen at the beginning of the film.
Keri Lynn Pratt http://www.imdb.com/name/nm0005325/bio actually has this in her resume: Played a 17-year-old in an episode of the sitcom "Stacked" (2005), but she was actually 27 at the time. Her boyfriend/ husband must be the luckiest paedophile in the USA.
[EDIT] How about moving my releases in the general section, Gracie's Choice and Cat Girl Kiki, to the respective release sections?