I think our DD was 6 when she finally demanded the truth. She'd been thinking about some of the various impossibilities for some time, and as she'd explore it out loud with me, I'd be waiting for that epiphany, but just when it was about to happen, she'd drop the subject like a hot potato. As usual, the moment of truth came when she'd heard from her friends, and since she now had peers who knew otherwise, we didn't want to turn her into a social outcast for believing bizarre things (we're quite content to turn her into a social outcast for believing things that are true in our tinfoil-hatted part of the country).
When DD finally came to it, she was not to be distracted by, "What do you think?" Her response was, "NO! TELL ME!!" She was very disappointed, and we could literally see some of her innocence disappear.
A couple of months later, she decided, "I want to believe." She made out her Santa list, and she had the traditional mall photo.
