I don't know what counts. My first sexual encounter was with myself. I played with my penis for as long as I can remember.
I also shared a bedroom with my older brother and we played with each other almost every night. He had a tiny penis, I had a big one. His hands fit my penis and I could jack him off.
I was involved in some circle jerks while in high school. Just our own cocks. No touching others.
After high school I made a trip to San Francisco to see a friend and we masturbated each other to orgasm. That probably counts. I was 17.
By 18, I found a woman to fuck. After that there were a number of sexual encounters. Both genders.