children with normal hearing-- spoken to in sign-- by deaf parents, are a little slow with verbal skills, but the difference is gone by age 5-- they learned language.. (sign language) and have "primed their brains" for language.. and when with verbal children, quickly pick up the spoken language...

It is well-documented that infants in signing homes are able to produce signs long before infants in speaking homes can talk. My son (who turns one next week) had seven signs in his vocabulary at 6 months and began signing two word sentences at 10 months. That they get "language" is clear, and because they develop motor skills before those tough larynx skills, they can produce language a bit earlier.