I was four; my mom routinely got my sister ready for school, picking out her outfit, fixing her hair, every morning. This took quite a while, so, feeling ignored every day, I went to my sister's room and took one of her dresses and put it on, went up to my mom and sister, and proudly said, 'I'm ready to go to school, too!'. Of course mom would have none of that, and got me changed back to my boy clothes, and we walked my sister to school.
When we came home, mom asked me if I really wanted to wear the dress, but said it in such a manner to imply that my answer should be no, so that's what I said. I hadn't seen myself in the mirror, so I don't know exactly what I looked like.