I had that talk with all 3 of my children (well, let's say talks as it is a continuing, evolving process) - including my 2 girls. For whatever reason, my wife found it difficult and left the sex ed up to me. I started talking to them about all of this when they were as young as 7-8 years. About the time that they started noticing anatomical differences between us (wife and I) and themselves. I think that by starting these talks early on and keeping them age appropriate (simpler basic facts to start with) and adding more info (what swear words and generally inapopropriate terms mean) as they are ready and capable of handling it made it a lot easier for them to transition into adulthood without being traumatized or stigmatized by "not knowing" what was going on araound them.