This is a site that describes a number of research studies showing the benefit of using sign language in young children. Most children don't start speaking unitl 18months or 2 years old. the most important thing is to use both verbal language and sign language together.
My 2 year son was not speaking more than just mama and dada. We started a toddlers sign language class and he started communicating with signs right away and was speaking all the words to match the signs within a couple of months. As his verbal language grew, we stopped using the signs.
take care, S.