It isn't a counter argument.

Yes it's unsafe. But nagging / pestering them on it does not promote online safety and is hypocritical.

Not to mention, there are kids under 15 here, and no one has said a thing. So why now?

Not saying it ISN'T dangerous, because it sure is, but like I said, you are not a 'parent'. The kid will either listen or they won't. So just ignore and block if you are uncomfortable.