I don't have the greatest memories but according to others, a lot of things were happening way before my memory could kick in. Earliest age I do remember abuse, I think I was around four or five, but it's so blurry and far away I can't tell. For certain I remember things at six and later.

I think babies can understand enough to know that it's bad. It taps into primitive instinct and therefore a baby can learn from it, even if the memory escapes through the gaps of time.