So, at least a third of that increased slope is due to increased crash frequency. That leaves a lot of increase due to such factors as reduced mental acuity, longer reflex times, increased opacity of ocular systems, degeneration of vision, and other age-related physical maladies. There comes a time to hang up the car keys. One can do it gracefully or lethally, but it will be done.
Among older drivers, marked excesses in crash involvement did not begin until age 75, but explained no more than about 30-45% of the elevated risk in this group of drivers