I'm not sure what age dd was but probably around 7 or 8. She was told at school by another child, but didn't let on to us and carried on pretending, possibly for our sakes. She did the same with the tooth fairy. Though even now, she won't actually say she doesn't believe in them; I think we all collude in the pretense that they exist simply because it's more fun that way.
I know I was about 6 when I realised Fr Christmas wasn't real and that was from school too. My elder bro had apparently known for years but had kept quiet about it so I and our younger bro could continue to enjoy the thrill of Fr Christmas coming down the chimney (we had them in every room).