I worked it out as soon as I discovered that trans men were a thing, and that not all trans people were women. Sometimes I wish I'd worked it out sooner, because all the signs were always there, I just never knew it was possible, and every single person I met was telling me that I was female, they all couldn't be wrong, could they? All female born people must just feel this way? Nope, wrong.
For a long while I regretted not coming out sooner and living my life as it should be lived, because everything clicked into place, and the irritation and pain that I never even knew was there to that degree before was lifted, but I've come to terms with it now. I know that coming out while still living with my parents would have never done me any good, heck, my father hasn't spoken to me since I came out, so it's worked out brilliantly that I never have to see them again.
There's no use dwelling on the past, nothing can be done. All we can do is use the knowledge we now have into making the present and the future the best that it can possibly be.