Legal Question in Criminal Law in California

How young is this "teen," what is the nature of the relationship, and what do you consider "having sex?"

A person who is 18 or older is an adult, so they can have sex if they like. However, sex with a 13 year old is considered a violent felony.

President Bill Clinton famously said, "I did not have sexual relations with that woman," but it later turned out that he had oral sex and other sexual contact with a White House intern. Prohibited sexual contact with a minor includes many types of conduct, not just intercourse.

generally no ... but Mr Marshall's advice is sound ... depends on how you define that. If zero nakedness or sexual touching of course sure an adult & under-18 age can hang out all they like ... same as with children/parents, grandkids/grandparents etc. Good luck!

It is a crime for an adult to have any kind of sexual contact with a minor. The law does not only forbid intercourse. Even some activities that don't involve physical contact are forbidden. If these two are having any kind of sexual contact then the adult is committing a crime each time it happens. But if the relationship is truly platonic then the law does not forbid it.

Of course, whether the relationship is illegal is not the same thing as whether the teen's parents approve of it. They have the right to tell the adult to stay away. And they may be able to get a restraining order against him if he refuses.