I think that graph is a lot of dodgy stats (not entirely but largely).
Many people drive in a manner where they attract and induce accidents but since they do not technically cause them, they get away scot free.
I'd imagine teenagers crash a lot due to a lack of experience, not knowing how to drive defensively, and occassionally due to recklessness inherent in youth
But old people get into a lot of accidents because they tend to drive crappily, often excessively slowly, but people tend to drive into them instead.
Also a lot of miles get driven by various groups of commercial travellers but inevitably many end up doing a lot of their miles on the open road.
Similarly, in flying, you might have the following very different situations:
- short trips in good weather, VFR
- long trips in good weather, VFR
- long trips in good weather, IFR
I'd say the last one is safest per mile. But it won't be the safest per trip, not least because those are mostly high-hour pilots.