I can't remember the first time I felt the urge, but the earliest memory I have of it happened like this:
Early 1990's. My parents owned a thrift store. I was probably 6 or 7. The people who ran it had grandkids who were the same age as my brother and I. The place was really huge. Lots of clothes and other items. We were playing in stuff one day, and I suggested we dress like girls. So we did. I remember it being really fun and exciting, and the other boys thought it was great. My brother never mentioned it to my parents, and I don't remember ever talking about it again.
I've been re-watching the Simpsons recently on Disney+, and there's an episode where Bart and Milhouse do the same thing. I remember seeing that when I was a kid, and it makes me embarrassed/ashamed now like it did then. I hate that. Kids shouldn't be ashamed of things they want that don't hurt others.