I was five. My one older brothers told me. Then all three of them "realized" that if mom knew I didn't believe in Santa Claus we wouldn't get gifts from Santa anymore. So they sat me down and explained to me that I could never tell my parents I didn't know about Santa Claus or I would make my mom cry and we wouldn't have Christmas anymore. My mom thought I still believed until I was 14 and she still gives us presents from "Santa".