The ratio formula IQ = (mental age / chronological age) x 100 works reasonably for children and breaks down entirely for adults. Why does the arithmetic fail at the top of the age range?

C. Because mental age plateaus while chronological age keeps rising.

The formula assumes both quantities keep climbing together, which stops holding once development levels off in the late teens, so the ratio falls for every adult alive and reports an unchanged 60-year-old at about 30. Wechsler abandoned the ratio: deviation IQ asks where you stand in your own age group, which is why modern scores still mean something at 60.