I'm of the mind-set that you're never too old.
I use the "Signing Time" series. You could start with the "Baby Signing Time series". If you check your local craigs list, you may be able to purchase the complete Signing Time series for a really good price.
It's a great program, it's Audio, visual, musical. My almost Two year old can say all her letters and can identify them because she liked watching the "ABC" DVD. LOL
Here is a good Link I use too:
http://www.aslpro.com/cgi-bin/aslpro/aslpro.cgi