In my case I was a 'late bloomer'. On the one hand on an unconscious level I've known since maybe I was 10yo that I was 'different'. At that age I was completely fascinated by concentration camps and nazi horrors completely oblivious to the suffering of others. I even made fake uniforms for my friends and I to wear. But it was just a growing phase and I didn't grow up into a white supremacist asshole.
To cut a long story short when my grand-parents died I then fully realized I didn't have any empathy at all. It was the turning point for me. I started reading about the condition and even though I'm not diagnosed I tick so many points on Hare's checklist that it's as if it was made for me.
What about you?