She's 3, and ASL was her first language. She was signing "mama" at I think 4 months old (I have a picture and everything.) She knew 250 signs at 18 months old, and could communicate all kinds of interesting stuff. (Like the urgent message that there were orangutans -- "orange gorillas" -- that lived in our tree.)
Her ability to communicate so early (babies can manipulate their hands far earlier than they can speak) really cut down on her frustration level, and she never really had "the terrible twos" -- I mean, not implying that she was a perfect angel (she WASN'T) but communication really helped.
She said some words early on, but didn't really start speaking until fairly late -- 22 months? I remember lots of conversations that went like this; "Does she talk?" "No, but she signs over 250 words!" "Yes, but does she
talk?"
Then she started talking, and is now really articulate. Multi-syllabic words, perfect sentence construction, etc. She's also tall for her age, and people regularly think she's a shortish 4 year old because of her speech.
Anyway, enough bragging, but she's a smartie. Teaching her ASL early worked out great for us. (She still signs, usually according to situation -- if she wants to say something "secret" in public, if her mouth is full, etc. I lipread, and can usually understand her speech.)