Interesting question
If children are born into a sign using household they will acquire sign language as their first language regardless whether the children are hearing or deaf. American Sign Language (ASL) is a complete, natural, rule-governed human language and as such babies and young children are "pre-programmed" so to speak to acquire whatever language they are consistently exposed to. It is a natural process. Deaf or hearing children surrounded in their early years of development by the use of ASL in their households and by significant other adults will naturally acquire ASL just like any child acquires his/her language. How human infants and young children acquire their mother tongues through natural exposure is quite remarkable as a phenomenon but unremarkable at the same time. It just happens through exposure, meaningful adult to child interaction etc. If you read about the natural development of human language in infants and young children you will know exactly how it happens for ASL. It is the same process.
Regards
Bill
