If she has partial hearing loss, what is it from? Is there a possibilty she could loose it in her other ear? Is there a possibilty she could get full hearing back from surgery or other treatments?
If she could loose all hearing, you should start as early as possible. On my birth board, people have suggested about 3-6 months as a good age to start. I'm going to do it with my hearing son.
I think knowing ASL is good at any age. Knowing another language is always beneficial. Your daughter is in a unique situation because if she learns ASL, she may have many more opportunities later in life.