If I wanted to be cute I'd say I was ~born this way~. Although that is true mind you.
To be serious I sort of definitely knew sometime last school year. Like, I always appreciated men more but I never really thought I was sexually attracted to them. Then sometime when I was 14/15 I was like, ok, I think I like dudes. Pretty harmless, right? Although for the longest time I tried to convince myself that I could be bisexual just so then I can pretend and fit in. Didn't work. So yeah, when I was 16 I knew for sure. I'm pretty glad that I know actually.