I did want to chime in on the spanking conversation, as a child who was punished corporally, it never really taught me anything other than "better not get caught" rather than "better not do this at all" which is what my parents intended. Small children don't understand that they are being hit as a direct result of something they did, they have already forgotten about it by the time the swat comes. I do use spanking and will continue to do so if something dangerous is being played with, like a kitchen knife. Otherwise it's taking away privileges and giving more chores, the worst punishment ever!
If the excuse against corporal punishment is they don't remember and don't make the connection wouldn't the same apply to all punishment and teaching?
My niece had my great niece speaking sign language long before she could make any kind of understandable attempts at words. She could scream in an attempt get what she wanted like most babies, or she could be told to calm down and sign what she wanted. They recommend starting baby sign language at 3-9 months.
I hear the same excuse about dogs not remembering to keep people from hitting them. It's wrong and can lead to a badly behaved animal that gets abused, dumped or staked out in the yard. Instead of telling people puppies can't remember I tell them the time to start training is as soon as you get it.