At What Age Should Kids Learn The 'Truth' About Santa?

As we enter the Christmas season, I'm facing the question that no parent wants to answer. My six-year-old has been asking me about how "real" Santa is, and I've kinda been tiptoeing around giving her a real answer. It's now my turn to figure out how to explain...you know..."the truth" about Jolly Old St. Nick. I keep telling her that if she believes he's real, then he's real. I'm trying to keep the magic alive for just a little longer (especially since I have a four-year-old whose Santa skepticism shouldn't kick in for a few more years).

She's starting to hear rumors from kids at school, but I just keep going back to "if you believe, then he's real". Am I protecting her magic in Christmas? Maybe. Am I protecting myself by avoiding the question? Maybe. I just don't want her to be disappointed and lose the magic of the holidays. I want to hang onto it as long as I can.