Signing for Babies

I picked up a book from the library on Baby Signs. I had read a lot of magazine articles and saw a show about teaching them sign language before they can speak so that they can communicate with you, but nothing ever said how old to start doing this. You don't want to start too early and get frustrated because they aren't ready to do it. They I got that book from the library and it says to do this when you baby waves good bye, because they have started to learn hand signals to communicate by waving. So, when is that? At 7 months. So, I have some time yet.