> How about 16 yr olds? 15? 13? 10? 7? Should we draw a line for "dipping their toe anonymously in adult activities" somewhere [...]?
Age of consent differs across countries. So yes, different countries and jurisdictions already draw a line for various adult activities. They also disagree what those lines should be and where they should be drawn.
Quite frankly: that line should be parents to draw and enforce, within reason. The liability should not be on the dating sites to prevent it from happening, not should they be forgiven for ignoring it when it does happen and it's brought to their attention.
I'm in the unenviable situation of having had one of my big projects shut down for precisely this reason: it's too expensive to moderate sites that kids may use. The reason for that is actually very simple, proved by data, and verboten to say.
Age of consent differs across countries. So yes, different countries and jurisdictions already draw a line for various adult activities. They also disagree what those lines should be and where they should be drawn.
Reality is messy.