It was written in the 1930's with the classic Gershwin music. It is largely successful, though some "some critics from the outset have considered it a racist portrayal of African Americans."
It tells the story of Porgy, a crippled black man living in the slums of Charleston, South Carolina. It deals with his attempts to rescue Bess from the clutches of Crown, her violent and possessive lover, and Sportin' Life, the drug dealer.
When I was the age of your daughter, I saw Cats and The Sound of Music. I think, that the themes are mature, but you aren't going to have too much language and such going on given the time period the play was written, unless it has been modernized... However, I haven't seen it so I don't know how much violence towards woman or drugs are prevalent. I mean, I loved West Side Story, Grease, Guys and Dolls and A Streetcar Named Desire at that age, and those have a gang fight and death, as well as a possessive man attacking a woman, sex and such in them, though mildly.
Wikipedia generally has a decent summary of plays you can check out in the future.
http://en.wikipedia.org/wiki/Porgy_and_Bess
