Quoting Autumn Whisper:" It is not "the talk." It is an ongoing conversation about their bodies, care for their bodies, and respect ... [snip!] ... that starts young and doesn't really end. Questions should always be answered promptly, honestly, and age-appropriately."
My son is 3 and I explain our bodies and how they work fairly often. We haven't gotten to sex itself (DH and I have been on pelvic rest for months, so it hasn't come up since i got pregnant, but he was curious as to what was happening when he almost walked in on us a couple of times right after DH got back from deployment before we got our BFP). I tell him that touching himself is fine and we talk about "private parts" and he knows the medical terms for all his parts and the girl parts too. He knows they're private and only he can touch them unless he needs help from MOMMY or DADDY to wash them and wipe his tush (no one else). He knows its rude to keep your hand in your pants 24/7 (his new habit) and that if he wants to play with himself he has to do it in his room or in the bathroom with the door closed.
He knows that mommies and daddies do a special kind of kissing (tongue kissing) and that is not appropriate for him because he's not a grown up yet.
He also knows what a period is and that a baby will come out of my vagina and she will have a vagina and such.