When she's curious
You want her to learn from someone she knows/trusts, rather than some kid who misinforms her on the bus, right? Who better to teach her than a loving parent, who can also share values? Don't wait until she asks; you can also bring it up when you watch tv/movies together, or when topics arise in the news.
(un)fortunately, there are many opp'ties nowadays. There are also helpful books.