Regardless of how old you are, YouTube will vet you based on what AI concludes from your viewing behavior. And, if it decides your behavior has been, well, juvenile, you may have to provide ID evidence that you are an adult. Other criteria such as longevity of your account will also be taken into consideration, but this is part of broader trends of automating age verification, banning young people from social media, and prompting privacy concerns over using data models to analyze user viewing behaviors.