My little sister didn't know Santa Claus wasn't real until she was something like 12 or so. She came up to my mother and asked, "Are you Santa Claus? Tell me the truth," and my mom was totally sick of the whole thing so she very gently admitted that yes, she was.
I chimed in and told my sister about how I learned they weren't real at a much younger age by deciding not to tell my parents when I lost my next tooth to see if the Tooth Fairy still came, and, sure enough, nothing happened, and I got to jump out at my parents the next morning and triumphantly declare victory over their good intentions.
But at this point in the story, my sister's face scrunched up, and she cried, "You're the Tooth Fairy, too!?" and then stormed off. Oops.