Legal Question in Business Law in California

Depends upon what you mean by "hold up in court."

Being under 18 would not, in itself, be a defense to a lawsuit. It might be a factor in conducting the defense of the lawsuit. For example, a three-year-old was once found not liable in damages when sued for pulling the chair out from under her aunt, who was about to sit down in said chair, and who sustained injuries when the receiving station for her rear end turned out to be the floor. The child's defense was that she was unable to understand that her prank might be dangerous. This wouldn't help a 17-year-old who published defamatory and untrue statements about another.

This says little or nothing about the merits of the threatened lawsuit, or whether it would ever actually be filed and served. Many, many lawsuits are threatened that never get filed. Also, if you are sued, you might win, or the plaintiff might give up part way through the suit, after deciding (for example) that you didn't have any money to pay damages anyway. Note: in some cases, parents are added as co-defendants on a theory of "negligent supervision" of their minor children.

Most likely, this person's anger is gonna subside before he or she spends the time and money to file a suit. However, if one is filed, don't fail to defend by filing and serving a response.