I imagine person B as someone who doesn't do formal exercise at all. If their capacity for exercise goes down, they might think vaguely that they're a little sick or getting older, but they've never been concerned with the details of how much they can do, and it's going to a good-sized change for them to notice it.

However, I'm not sure this pattern extends to reading-- some smart people who read easily seem to have the metaphorical proof-reading/copy-editing gene.