It depends, are they a minor? If they're a minor, I'd be worried about the fact that they're talking about something so intimate with anyone but me. I'd ask where they found that out from.
If they're an adult, then I think it's a no brainer, it is none of my business. To disown them at that point would just be petty behavior that exists purely for my middle-aged comfort, not to help my child. I wouldn't financially support it, but I wouldn't be disappointed. Partially indifferent, as at that point it's not my job to have an opinion, and partially happy that I managed to be such a reliable parent that they felt comfortable even telling me. I don't even trust my parents with what my new favorite band is, so them being able to trust me with their "new gender", that's a silver lining that they truly see me as a source of guidance.
If they told me I'd probably ease the tension by joking about how "Oh, I've BEEN known.", that's what I did with my last friend that came out and it made the situation a lot less awkward.