I don't think anyone could argue that certain physical and mental faculties deteriorate with age - physical problems in general are fairly easy to assess and remedy, mental problems such as reaction time not so easily.
I have no problem with the idea that drivers should be medically assessed...
Extracted from Department for Transport official statistics for 2012
Of course these figures don't tell the whole story - missing for instance are comparisons of mileage driven by each age group - they don't however support the idea that elderly drivers are involved in the most...
You are forgetting one other aspect to driving which is as equally important as vision, awareness and reaction - anticipation - something which comes with experience
I'm 66 and would have no problem, in fact would welcome, being retested at least every 5 years.
The vast majority of people claim to be good drvers - if that's the case where do all the idiots on the road come from. 20 or 90, very few recognise their own faults.