I'm fairly certain that legal action has only been taken in cases where the adult actually took steps to take the chat offline. Things like asking to meet, or asking for pics. Without that, it's just lewd comments, which are a misdemeanor, even when directed at a child. And if the person producing the pics claims to be over 18, but then sends pics of someone under 18, then they're the ones actually guilty of a crime (even if the pics are of themselves). So short version is, if you end up chatting with someone underage, just politely stay off of sexual topics if possible.