The Sandman - I knew I was different before I hit preschool. My mother recognized it when I was a toddler. As for physically transitioning, I firmly believe that everyone has the right to do it before 18, but not without counseling, both for child and guardians. You don't just hand someone the keys to a car without teaching them how to drive at 16. It's kind of the same idea. I would have loved to have been able to talk to a counselor about issues as a teen, but unfortunately, much less was known about gender identity when I was a teen, and I grew up in a small, rural town that wasn't exactly open and welcoming to people who were different. Some trans people are comfortable without ever having any physical alterations done. Some of us are not. it helps if you think of gender as a spectrum, rather than an either/or thing. And yes, I'm pretty certain that every trans person knows that they're trans before they hit the age of 18.