Probably when I was 13 or so. Before that I acted, well, not very straight, to the extent of kissing friends, who were all girls, and showing a rather superficial interest in the opposite gender, mostly out of peer pressure as I approached puberty. But somehow I didn't really realize it was weird until all my friends started crushing on boys unironically and I was confused. I also didn't realize I was crushing on girls until other people described to me in more detail what liking girls was like. I just thought it was normal friendship feelings (which is still difficult to parse from "liking" someone imo but whatever). My sexual awakening solidified it, but I was kind of in denial for a while. I'm quite happy now at 20, out, even if chronically single (mostly because of myself)