I was diagnosed in November 1999, I was nearly 13... (Two months off).
But its only been in the last month or so, since I've been on Wrong Planet, that I've really accepted it. My brother is severely autistic, and so I was convinced I had learned a lot of my behaviours from him. I still do think that to an extent, but I havent seen him since 2006 (He lives in Australia near my mother, I live in England) and I'm still exhibiting some of those behaviours and I'm like well Patrick's not here to influence me, so I must have it, but I feel like a fraud.
I would tell your daughter, and just say that if she gets a diagnosis, she doesn't have to accept it straight away, but if she takes too long to accept it, she could miss out on a lot like I did.