I'm teaching my 18 month old son right now. I taught a few on my own and we are currently taking a baby sign language class at the YMCA. I was concerned because he barely says 3 words, dada, mama, and uh oh. I have a meeting with a speech pathologist scheduled later this month to evaluate him to make sure there isn't anything developmentally wrong. The lady who came for the initial meeting was signing to him from the moment she stepped in, and she relieved my worries that sign language could further delay him (I had read repeatedly that that ISN'T the case, that sign language actually helps them, but I was hearing doubtful comments from family and friends.
Let me tell you, it is awesome! His mind is a little sponge and he is learning new signs almost every day! So far he knows bye bye, eat, all done, milk, book, diaper change, yogurt, banana, baby, more, help, ball, water, and dog. It is so cool to be able to communicate with him. If he is having trouble with something, he signs 'help', if he is hungry he signs "eat" and then the thing he is hungry for (usually bananas!).
It really alleviates whining and tantrums because the child can actually specifically communicate their needs to you. Non-signing babies can cry, whine, yell, etc. to let you know they want/need something, but sometimes it's a guessing game as to exactly what it is they need. Also, my son will just comment on things happening around him, so I know what's going on in his little mind. It really does open a whole new world!
I HIGHLY recommend baby sign language, you and the child will benefit! The key is that if they are hearing babies (not hearing-impaired) it's crucial that you always say the word you are signing; remember, the ultimate goal is to get them to speak!
The lady from MD's Infants and Toddlers program lent me a couple DVDs on baby signing called "Signing Time!"... they are EXCELLENT. Before watching them I thought they would be too boring for a little kid to learn from; however, they gear it toward kids with a combination of illustrations, a person demonstrating the correct sign, little kids & babies signing, video clips, kids voices, etc. It really holds my son's attention. I mainly work with him one on one and in his class, but occasionally let him watch the DVD, which he loves! There are a whole bunch of volumes... you can get more information at signingtime.com.
PM me if you want any more info on baby signing!
Laura
ETA: In response to your question of how young can they be, my DS's baby sign language teacher said she has had a baby as young as 7 months begin to sign. You can begin signing as early as you want to, just remember that 7 months is about the earliest they are able to sign back, and it could take longer. Don't give up! I didn't begin signing until Jacob was well over 1 and he didn't seem to take to it after many weeks of me signing. I got discouraged.
Then suddenly one day, after I had stopped signing for quite a bit of time, he signed 'book' and wanted me to read to him! That was such a great feeling! I ran outside in my pajamas to go tell DH that Jacob signed his first word! and that is when I went full force into teaching him more signs and signing him up for class.