I think one's definition of boyfriend changes with age. In kindergarten it was who played with you, in 3rd grade it was who paid attention to you, in 6th grade it was who asked you to dance, in 10th grade it was who you spent all of your waking hours thinking about, after high school it was more short term until Mr.Right came on the scene.
I was 12 about to turn 13 when I had my first "bf" it was more for the name than the actual thing. We didn't do anything. My first real boyfriend was in 9th grade. He was my first kiss, we went on dates, held hands, hugged, hung out, talked. I was 14.
Well probably when I was in kindergarden but that wasn't very serious. A girl named Sarah stole him from me. Probably in 5th grade and it lasted close to 5 months but we broke up before 6th grade started