Probably around 11-12. And I agree with Millionaire - it should be more a series of conversations than one big talk - that way, he knows he just needs to ask.
I have one thing I'd like to add, and please take from it what you will, but I feel it needs to be said. Please note I do not know you, him, or your religious beliefs or other feelings on sexual matters. I speak only from experience.
Please do not just assume he will have a girlfriend. Yes, the sex talk is most appropriate focusing on a heterosexual relationship, as MOST people are, but not all. And by focusing solely on "straight" relationships, you may make him feel as though, if he wasn't straight that there is something wrong with him, and may isolate him further IF he isn't straight.
It doesn't have to be an obvious "well, if you're gay/bi etc and bring home a boyfriend" type statement, but the use of partner rather than girlfriend...or something.
I am not saying he is, or could be, but it doesn't hurt to have an open mind - that way, he knows, no matter what - you love him.
I do applaud your willingness to talk about it, and to also focus on the 'respect' issue - most people are so 'icked' by talking to their kids about sex - they focus on the "physical" nature of things, but not really the emotion and the other person.