Answer from the comments.

Since $\sqrt{40} = \sqrt{4} \sqrt{10} = 2 \sqrt{10}$, the difference is:

$$5.8 \cdot 2 \sqrt{10} - 5.8 \sqrt{10} = 5.8\sqrt{10} (2-1) = 5.8\sqrt{10}$$

which is $18.3$ million, rounding down to the nearest tenth (of a million).

This answer is the right order of magnitude as there are around $330$ million Americans, so the change in this age group should be an order of magnitude less.