How is touching illegal when one person a minor and the other not?

Touching is not illegal simply because one person is a minor and the other is not. There must be a specific legal basis for the allegation that the touching was illegal. For example, if the touching was sexual in nature, it could be considered sexual assault or statutory rape, depending on the jurisdiction. If the touching was not sexual in nature, but was still harmful or offensive, it could be considered assault or battery. In either case, the specific laws and circumstances of the case would need to be considered to determine whether the touching was illegal.