I never got upset or disappointed when I found out Santa didn't exist. My parents went the whole hog with it and I absolutely loved it, they still protest that they believe in Santa and they're in early 40s
I eventually worked it out when I was about nine or ten, after telling people off for saying he wasn't real for years - not encouraged by my parents, I was just so convinced he was real. They used to tell me "My mum buys my presents", I told them it was because they were too naughty to have presents from Santa and their parents just felt sorry for them 
I pretended I believed for a couple of years, enjoying the grown up feeling from it, and then when I was about twelve, I went to my mum and said, very solemnly, "Mum, there's something I think you need to know. I know you've believed in him your whole life, but I've done some research and I don't think it's possible for Santa to exist", I was terrified I'd be breaking her heart
as it is she said she feels that he exists in people's heads for as long as they believe in him, and there's nothing wrong with keeping that belief going if it makes them happy. My brother and sister were both the same; we kept it going for them and then gradually they've figured it out (my sister bless her didn't realise until this year and she's twelve), but neither felt hurt or betrayed, despite my parents going quite overboard with the santa thing - always putting out cake and carrots, doing the flour footprints on the floor etc. We've never been bullied for believing in Santa beyond six or seven - or if other people saw it as bullying, we must have had thicker skin or something because it bounced off, as we knew we were getting presents from Santa 
I can't believe some of the people on here who consider it a betrayal to their children and "lying to hurt them", how ridiculous
it's a bit of fun, and if people were really that hurt by it as children, someone would've made a fortune by now setting up a business as a Santa Betrayal Counsellor
I'm so excited for doing the whole Santa thing with my child, and I think it's one of the things my parents are most excited about too - just as their youngest child gets 'too old', they get a grandchild to do it all again with 