Full sex talk... depends on what you mean by that. Going into all the knitty gritty details? I would probably wait until early adolescence for that one, but I think talking to them about reproduction and how they were created can be told to them in simpler forms at younger ages.
I would start giving them some form of talk as early as possible if they show a curiosity, otherwise just before puberty and at puberty I would give them the proper talk.
Is it so shameful to talk to your kids about sex. Are you ashamed that you had sex in order to create your kid? I don't understand.