Elf on the shelf is a popular holiday tradition where an elf doll is placed in a household to monitor children's behavior and report back to Santa. While there's no definitive age at which to begin this tradition, it's generally recommended for children who are old enough to understand the concept and the purpose behind it. It's typically suitable for children aged 3 and up. However, it's essential to consider each child's maturity level and how they will react to the elf's presence.