I don't think it's necessarily too young to begin asking questions about sex and body parts - after all, a lot of girls are starting to menstruate at age 9 and up, and she's just entering those years...

Simple, but honest answers are the best bet - if she wants more details, and you've answered her questions honestly and without embarrassment, so she knows she can trust you, she'll ask for more information when she needs it. Make sure you use the proper words for body parts - penis, vagina, vulva, etc. - teaching her these will make her a less appealing target for sexual abusers, as they tend to be drawn to less mature children; and it also makes life easier should she ever need to see a doctor about these body parts in the future.