I don't remember, I do know that the adults (ISFJ step mom) seemed to enjoy the whole "santa" thing more than we did, we still get presents from "santa." I was the 4th of 6 kids so I think I knew earlier than I probably would have without older siblings.
I knew someone who once was embarrassed because their parents told them that santa was real and they argued with people at school because their parents would never lie to them. I tell my kids around kindergarten age because I don't want them to be embarrassed about that kind of thing. I also think any younger is a bit confusing for the kid, they aren't really old enough to get it either way.
I just asked my 5 year old preschooler (ESP - 4th child) "who is santa claus?" "You" "Who gives you presents" "You" "Who came to your preschool as santa claus" "You" (It was a male and I was there taking pics) This is why I don't talk to preschoolers about Santa.