I bet it has to do with laws around processing of data for children.

From the ChatGPT terms:

> Minimum Age. You must be at least 13 years old or the minimum age required in your country to consent to use the Services. If you are under 18 you must have your parent or legal guardian’s permission to use the Services.

Source: https://openai.com/policies/terms-of-use

Apparently it's just GPT 3.5 which is freely accessible anyway. If it was GPT 4 / ChatGPT Plus, I would understand.