I'll lie or tell them half-truths until I feel they're old enough (and equation that factors in age, their maturity, and what their peers are saying at school, so probably 4th-6th grade), then I'll tell them everything straight up, biology-wise and safety-wise. Well everything except the various positions and tips and tricks like how to find the g-spot (which they should be able to do on their own, jeez it's one quick Google search away, how is it a mystery?).
That's what my parents did for me and my sister (who's two years younger then me) when I was in 5th and she was in 4th grade. For some reason they're letting the school teach my younger brother, who's in 6th grade now and still clueless. I wish he'd hurry up and find out about it so I can tell him he was a mistake (he is 10 years younger then me, certainly not planned lol)