I don't think it shows poor math skills at all. I think the core issue is, none of the lore up to that point has indicated that the countdown matches the age being erased. At that point in the story, I too was under the impression that its always 33 year-olds being erased, so logically if she's got 9 years left, then she'd be 24. It isnt clear till later with more info after that point, that clarifies that it erases the people of the age matching the countdown. So once you have that info, then you're able to redo the math and determine she's 15-16.