The longer story
Most mainstream AI chatbots set a minimum age of 13 in their terms of service, and several ask for parental consent for users under 18. Those limits are a legal floor rather than a recommendation. The more useful question is whether your child can recognise that a confident answer can still be wrong, and whether they would tell you if something they saw upset them. Younger children get far more out of these tools sitting alongside an adult than using them alone. Check the terms of the specific tool your child wants, because they vary, and some now offer a separate under-18 experience with tighter safety settings.