I think whenever a kid asks is when s/he is old enough to be told the truth. My son is 8 and just asked for details this year. We told him about the egg several years ago...but it took him this long to think to ask how it actually gets fertilized. I took the opportunity to discuss condoms (I called them a special cover) as a necessity when he is old enough. That got into a whole discussion of whether it was a law to wear a condom (should be, ha ha) and I said no, but it is my law for you!
So we are doing it a little at a time. There is no big talk, no major discussion, just me answering questions as they come up.
