I don't remember how old I was, but my daughter found out last year at age 7. She asked me to be straight forward about it, I couldn't pull it off and a huge part of me didn't even want to try.
Sorry guys, I've never been in love with the notion of Santa. Which reminds me of..
"Santa. Let's see, what have we got here? We've got an S and an A, an N, a T, and another A. Hmm.. Who could be causing all those laps to bounce up and down curiously? Who would help grown men peel the focus from the baby Jesus on his birthday? Who could it be, I just don't know. Could it be.. Satan!!
but of course, Santa lives in all of us, etc. We're each others Santa.