1) thinks that voluntary pedophilia is a thing
2) children can consent to sex with adults (again, broad statement that he didn't even limit to older teens)
That's very different from children experimenting together. Stallman implies that not wanting some adult creep to prey on your kid is just some sort of moral panic, which is... a pretty horrible take and one that is often repeated by predators. A teen having sex with another teen is normal and part of the process of maturing, but with an adult it has a completely different power dynamic.
Plus, being attracted to prepubescent or pubescent teens as an adult is inherently awful and problematic. It has nothing to do with a teen being attracted to another peer.