As I've mentioned elsewhere in another thread, I knew I wasn't right from the moment I could differentiate male from female and knew I was really a boy, except without the equipment, by the time I was about 6. I didn't quite understand, however, that you didn't just grow the necessary boy parts as you aged, but eventually clued in.
I think ages of discovery and re-action are different for everyone, but I suspect for most it's early on and I think this might depend on upbringing. The more repressive the upbringing, the later you realize all is not right. But I could be wrong. Ze? Thoughts?
Regardless of what age someone is when they finally come to terms with who they really are inside, they deserve our support and encouragement.
Your partner is blessed to have you and I've no doubt you'll be integral to helping, healing, exploring and some wonderful times ahead as well.
Best, always.