My DD had "the talk" in the autumn, and she was 10 in October - I can't remember how it came up, but she was asking questions. We went into full details, got the kids' encyclopedia out for diagrams, and she seemed happy by the end of it! Her younger brother was out somewhere, so he missed it - have to do that later, if she hasn't already told him!
That said, I'd told her about periods a year or two ago - following on from the embarrassing questions in public loos ("what are tampons, mummy?", looking at vending machine - I remember doing exactly the same to my mum ).
I've bought her a book (from Usborne, I think), explaining all the growing up stuff - she says she doesn't want it yet, so I'm checking back with her every few months to see when she's ready to face it all!
She's quite a young 10 year old, I'd say - not into all the teen stuff at all yet - which may make a difference.