Obviously I'm not a lawyer. But what I'm thinking is that the terms of service for the apps probably state 18 as minimum age. Discounting that violation-then it seems to me that talking to the person online depends more on state rules because using the apps could be considered a substitute for texting. And if legal age is, for example 16 in Washington state, then in general with some exceptions, it should be legal to discuss things.
On the other hand, knowingly talking to a 16 year old for hooking up is not something I want to do. So I'd block them.
Gman