I actually thought about how I was going to approach this with my first daughter if it turned out that she was biologically my child.
I asked my mother when I was 10 'cause I reached a point where I couldn't stand my father 'cause I believed he had abandoned me and my sister . She laid it all out for me in great detail and went a lot deeper in depth with me than she did with my sister due to my greater understanding of... well.. the world.
She went over their relationship, how pops married her in order to do right 'cause she got her pregnant, how he left, came back, fathered my sister, drove a cab and DJ'ed on the radio to make ends meet, etc only to bounce again. Eventually I asked pops why and he echoed the same thing my moms said.
With that in mind, I figured that I would probably have to answer this question with my daughter at around the same age, maybe as late as 12. Didn't really know how to approach it, but I always kept my parents words in my head so I at least I had that. It turned out that she wasn't my daughter and I haven't seen her since, but I have a very strong feeling that I'm going to come face to face with her in the very near future... And she's going to ask me what happened between me and her mother.
... And I'll have to go through with that speech anyways.