I would have to say either middle or high school, so mid to late 90's. I know I was fascinated with women's clothes since about age ten when I was accidentally gifted a dress that was meant for my step-sister. I didn't know why, but it felt "right" to receive such a gift and I was extremely disappointed when it was taken from me. Not long after that I had heard the term 'crossdresser/crossdressing' when my mother had found out I had been occasionally wearing her clothes... though they weren't used in a positive way. That was at about age 11 I think. As for "trans" words, around 15/16.