For me, this transition occurred when I was 12 years old. I grew up and lived most of my life in Mexico, but when I was 10 years old, my family and I moved to Madrid, Spain.
When I lived in Spain, all my friendships and activities were still very child-like. My friends and I would talk about school, sports, movies, etc; we rarely talked about boys or sex, and when we did, it was not a big deal. After two years in Madrid, we moved back to Mexico City and everything was different. I started going to a different school than the one I had gone to before I moved to Spain. Since I was in 7th grade, I went to a private escuela secundaria (Mexican equivalent to Middle school or Junior high). The majority of my classmates came from the same elementary school, but there were still many that came from other schools.
The first day of school was a cultural shock for me. In only two years, I had got very immersed in Spanish culture, and it was really surprising to come to a much different Mexico than the one I knew before. Everyone used slang Mexican expressions that I did not understand. For example, the very first day of school, I remember hearing a conversation among 4 girls, about how a boy and a girl in our class “se pusieron calientes” (became horny) but I had no idea what they meant by that. Now I do not think that they even knew what it meant, but we all pretended to understand and to be surprised. This expression, as well as many other things that I heard in this school, was so vague that it made me think that maybe kids my age were already having sex. It seemed to me that 12-year-old kids in Mexico were much more advanced than 12-year-olds in Madrid. In Mexico, most guys talked dirty and constantly made sexual jokes, while the girls laughed and said: “Ew! Stop being gross!”
After a year of making friends among these kids and getting used to living in Mexico, I became an expert in Mexican slang and sexual innuendo.
I do not know if it was because of cultural differences between Mexico and Spain, or if it was the different school system; or the huge American influence that kids in Mexico City are exposed to (by watching American movies, MTV, listening to American music, etc.); or if it was inevitable that I would have my transition at this age, but I think that my environment and contact with other kids were the mainly what made me stop being a child and become a teenager.