I started when she just turned 14 month.  My daughter also knows signing and I think that helped a lot.I believe that any kid can learn anything you teach him if you repeat it every day.

I just want to share this video. My daughter was 15 month old when we record this video. Now she is 16
<a href="http://www.youtube.com/v/6qv3XoPrLuM&rel=1" target="_blank">http://www.youtube.com/v/6qv3XoPrLuM&rel=1</a>

My daughter watches ONLY signing time for 30 min or so. Monday through Friday. Ocasionally sundays. Lately she has been watching Elmo. She likes it too much so I am not letting her watch for a while. Do whatever you think is appropiate for your child.

Hello
My experience was a little bit of each of you one. I signed with my daughter since she was 4 month old. No a real sign until she was 9 months and a half. And she had an explosion of signs after that. be aware that she will sign almost the same sign for some signs like butterfly, rabbit, monkey etc. But you have to be aware what she is seeing so you can understand her. My daughter won't speak any word but dada until the age of 13 months. When she turned 14 months she had another explosion of words. She is 15 months and she can speak up to 60 words English and Spanish and she also say a two word together like bye bye daddy or mommy. It's amazing. keep it up!