Here's what I found:
True it is that we over-70s are more likely to kill ourselves than the average of all drivers (See the column Driver deaths per 100,000 licence holders) but less so than than those 20 and under. When it comes to all road deaths - drivers, passengers, those in other vehicles, pedestrians and so on - (the last column) my lot do worse than the youngsters. Then again we do substantially better than all but the 40 to 69 age groups when you count all accidents with injuries (Casualties per 100,000 licence holders).
These figures were for the year ended 31 December 2014 and I suspect that with relatively small samples involved there are considerable short term variations.
I noted that in 14 fatal crashes sudden illness was put forward as a contributing factor. Perhaps that goes a long way to explaining the higher 70+ driver death rate.