Some may disagree, but I am waiting until they show some kind of interest in where they came from. This is what I did with my OE DD. I think she was around 4 and she is a very precocious 5 1/2 now and still hasn't asked about HOW EXACTLY she got in my body. She just knows that part of her comes from me and part from her dad. I was considering telling my boys earlier than that (they are almost 3) but I feel like I should instead just naturalistically respond to their questions rather than "teaching" them about their origins and making it seem like such a different thing. Parents don't have that talk with their OE kids out of the blue, right? If at age 4 or 5 they still aren't curious, then we'll pull some books out about different kinds of families so normalize the situation and begin a discussion. Right now they aren't even interested or have an ounce of understanding that they grew in my belly. They can barely sit still for a book. I think that at that age they will have a better understanding, too, of what family info to keep private. If I start talking about an egg lady now they are going to drop that phrase everywhere, and we are not tell to the entire universe.
Just my 2 cents.