It never made sense to me, but I believed it anyway. I think someone at school told me, probably a little older than most kids are when they hear it, like 8 or 9. Basically, I had always suspected it, but I think I remember rationalizing it away for one more Christmas.
It's odd, I'm not a very gullible person, and really have never tended to trust what people told me at face value. My parents must've told a really consistent story all those years.
The thing is, the year I found out, I also had a 4 year old sister, so I basically had to keep it going for her. As in, I had enough maturity to not run and ruin it for her (and my parents didnt even have to tell me not to), but at the same time I was still immature enough to have believed in santa claus until that age. Funny.
I remember when my parents finally told me a year later, and they were shocked to hear that I already knew but wanted to keep it real for my sister.