I started signing with my daughter when she was about six months old. She started signing before her verbal words began, of course, but when she did started speaking, her first words were one of our first signs ("all done") - and she signed it while she said it! Even how she said it is a testimony to how it works, because she was just over one-year old at the time, and kept throwing her sippy cup on the floor. After picking it up for her 3 times, I said and signed "all done," and then she raised both hands up like the sign and said "all da." She still echoes me when I tell her something in verbal or sign language, and I know she understands what I'm saying. She just turned 2, and is still using all her signs, and is still learning more (I have to look some new signs up for her because she gets bored with the same ol' ones) while her verbal language is growing exponentially day by day as well. (By the way, I used the book and video "Sign with Your Baby" to learn what I needed to help sign with her - it's an excellent learning tool and resource.) We have a new baby boy now too, and she signs milk for him when I'm nursing already, so I know he'll be picking up on it soon too! He's already surrounded by it (and that's how they learn it by the way - like any other language - just being surrounded by it).