First time I can remember pooping on purpose was when I was 8. I had done it before, in a diaper no less, when I was at my neighbors house and we were playing house. I usually played the husband while she played the wife, but this time she wanted me to be the baby and diaper me. I was only 5 at the time and saw no harm in it; and ended up pooping myself like a baby. However, like I said before, I was only 5 so I didnt know it was a taboo.
By the time I was 8 I knew that pooping in your pants was a taboo, let alone a diaper. But just randomly while I was playing Ape Escape on my Playstation I wanted to poop in my pants. I wore underwear back then so I put on 2-3 pairs, pull my pants up, squatted and pooped a very hard load into my pants. It was probably the most exhilarating I had felt in my whole life up to that point, and only one or two moments have ever topped it.