My brother told me when I was 7. Really made me mad at him for a LONG time. My own kids were 4 years apart. I don't remember how the older one found out about Santa, but we played the "Santa game" when his sibling was old enough. The younger asked me about it one day, and I explained it was a "game" that some people play because it's fun and anyone who gives a gift to someone else is a "Santa." When I taught second grade, I knew it would be an issue, so every November, I sent home a letter in a sealed envelope (so the kids couldn't read it) and asked the parents who were totally against the idea or whose children were already savvy, to ask their children not to burst another child's beliefs. If the parents had a problem with my letter, no one ever complained. This continued from 1980's-2010.