This via medicalxpress.com: “Most agree that the earlier you expose a child to a language, the easier it is for that child to pick it up. The same rules apply for deaf children.”
Research by Kearsy Cormier, Adam Schembri, David Vinson and Eleni Orfanidou.
The conclusion–from the article which can be found here–states:
“The current study supports many others showing that early exposure to accessible language is much more likely to result in successful language acquisition than later exposure. The advantages of early sign language exposure remain clear even with rapid advances in hearing aids and cochlear implants (Mayer & Leigh, 2010). Bilingual education is the best way of ensuring that deaf children have early exposure to both a signed language and a spoken/written language, which will provide the deaf child with the best chance for successful language acquisition, in either or both languages (Grosjean, 2001).”