I have to say, I'm a little sceptical of the idea of compulsory re-testing at 70. The implication is that if that 90-year-old had had another test, then that motorist would not have been killed.
Tragic as any death is, how is it possible to say so? The driver in question appears to have killed the poor motorist because he was suffering from dementia and had poor eyesight - not merely because he was old. Under Gromit's suggestion, the 90-year-old driver could have past his test at 70 and been driving quite happily for 20 years until his apparently recent diagnoses. Even to the best of drivers, accidents happen all the time. And 70 is a fairly arbitrary cut-off point - I fail to see why a 70 year old, say, is more dangerous than a 69 year old, or a 65 year old, etc.
It makes far more sense to have some kind of measure (I don't know what) for preventing people from driving after they have been diagnosed with dementia/eyesight problems/anything else which severely impairs driving. A compulsory re-test at 70 just seems wasteful.