Linguistics and neuroscience studies generally show that language acquisition and development in sign language L1 (as a first language) are on similar developmental timeline as spoken languages.
Scan language developmental milestones in Ausralian Sign Language (Auslan) from birth to 8 years old. Excellent detailed descriptions of bilingual developmetn scale in English and Auslan.
Levesque, Elizabeth (2008). "Bilingual English and Auslan Development Scale." http://www.aussiedeafkids.org.au/bilingual-english-and-auslan-development-scale.html