More like, you take a teen who's not adult yet, move it into a child's even more undeveloped body; what do you get? A child with experience.
Brains don't even finish developing until the age of 26, which means they can't possibly even function or think like a proper adult even in their original bodies.
You can't just lump the years they've been alive and add it on as a sort of mental age... you have to think of their brains' current development if you want to go down into technicalities.
But then again, Idk if that's what this author has in mind or not, I just don't think it makes sense otherwise.
Plus, being an abused victim their brain is heavily impacted more than you think. Still not sure how deeply the author is thinking about things, I do like overthinking things myself...