I was 12 years old during the Soviet era. I randomly saw a short vignette about computers in a pop-sci film on TV, and it clicked right away. I knew instantly that this is what I'm going to do for life. I had no idea if I'm built for this, or whether I'm smart or not -- and I didn't care. The books and projects came years later.

BTW, my dentist of 20 years is the same. At twelve, she knew that she will have her own dental clinic -- and she did open her own clinic about 10 years ago (she previously worked for hire). She's the best dentist I've ever had, and even though she doesn't do much work herself nowadays, her hires are excellent.