I don’t think we ever really told them. We just let them figure it out, which they all did, probably by age 10. I guess my advice (father of three) is to not emphasize Santa after age 5, to not be the first to bring it up, but to be honest when they start asking questions. I’d also emphasize the “magic” of the season regardless of believing in Santa or any specific religion. It’s nice to set aside a time of year to visit family and friends, and to show how much you care for them.