How on Earth is this supposed to make one bit of sense?

The exact same activity is healthy or harmful solely depending on the age of one's partner, which may not even be easily determined! It requires a belief in magic that transforms each person on a birthday to be determined by their local government.

Having thought about this a lot, I don't take the common line (imagine a deep God-like voice), "Children can't consent!" I think the relevant ages are a pragmatic matter, and adult-child sex with a large age difference is particularly likely to end badly for the younger party, especially if a girl. It's a difficult problem, and ultimately I leave the specifics to the teleiophiles (ordinary non-pedos) to hash out based on what's good for kids.

No magic. I even tentatively take the view that as long as a younger party is adamant that nothing bad happened to them, there should be no prosecution.