That chart says no such thing. What you think you see is only an artifact of how that data is presented. It can be presented as a line chart, which is what you quoted, or as a bar chart:

(the numbers approximate what Closetbiker presented in his graph; they are only approximations, and should not be treated as anything more than an illustration of the differences between line charts and bar charts.)

When presented as a bar chart, the data looks different, and you can see that for a ten-year age difference there is a difference in the risk. But it does not show where in that ten year the risk increases or decreases. It is not an instantaneous thing, and without seeing the numbers behind it, how they were derived, etc., there is no way to say whether there is a year-to-year decline or rise.

And, this is a straw horse as Achoo states, as it says absolutely nothing about bicycle helmets or bicycling fatalities and injuries. It is overall for head injuries.

John