The hypothetical here is that the child sought out the adult, not the other way around, and the child is near legal age and presents as if legal age. Given that set of assumptions, how much liability does the adult have?

That is the philosophical part of it. But reality isn't that neat. Here are some questions that need to be asked:

• how did the child get there?
• how much did the adults know? How much should they have assumed?
• what kind of pressure, implied or otherwise, was the child under?

I don't think Stallman considered that, I think he only considered the hypothetical.