Proof of birth to work?

I was talking to a friend who's 16, and I was asking him if he could possibly help me get a job at the place that he works at (Taco Bueno), but he said that I had to be 16+ to work there. He also said that I could possibly get away with it, as they don't ask for a birth certificate, only a social security card, which I didn't believe, but it got me curious. So, is this true, do you not need a birth certificate to hire at a fast food restaurant?