Birth! Seriously as soon as they can reach there and feel that it feels good they'll keep going back. I would gently move their hands out of the way during diaper changes so I could put the new diaper on and to keep their hands out of any messes. As they got older (I have 12, 7 and 4 yo ds's) we would tell them that it's fine to do that, we know that it feels good, but that it's something that is private and they should only do it in their bedroom or the bathroom (or somewhere private). At a young age it's not sexual, it's just physical. It's like scratching an itch or massaging a sore muscle. They don't know it has a sexual function or connotation, it's just something that feels good.
at 12:10 AM on Aug. 11, 2010