Judging by two elderly US presidential candidates, cognitive ability does generally decline as one its into the older yers. So, as an absolute, I don't think it I ageist per se. However, as I have suggested on the other forums, I think, like the aviation sector, drivers should be tested periodically. OME pointed out the logistics of the number of drivers divided by the number of examiner would be difficult to achieve. But this could mean, like CAA (and I presume CASA) class 2 medicals, at younger years the testing is further apart (4 years) and as one ages, it drops to 2 years and for me, now yearly, with an ECG every year (under 50 and its an ECG every two years). Maybe the test should be for cognitive ability in the same way he have period medicals for flying. but in GA world, are Australian pilots still subject to the BFR? Over here it is 12 hours of flying in the last 12 months and 1 hour instruction (they can't fail you - just recommend additional training) within 90 days of your 2 year anniversary. At least that's what it was in EASA.. not sure if the UK has since Reverte to BFRs... I am getting back into flying after a 3 year break or thereabouts, so I have to do a renewal.
I dressed.. for cars, maybe up until 50 or 60; once every 10 years, then from 50 to 70 once every 5.. Would be driven by the medical stats of the population of drivers.. there will those In their 90s that have full faculties and good cognitive abilities, but I would imagine that is not the norm.
Loss of cognitive ability, I would guess is also not linear. So, I would think a driving test over 85 once and nothing is pretty useless except for that point in time.
FWIW, I think the road law, fairly easy to test online should be done every so often. Today, a newspaper ran a quiz to see hoe much we retained of the road rules... I last did my road rules here in about 2008; I got 3 out of 15. Thankfully my daughter has the latest books as she is getting her provisional licence (L's), so I will be reading up to see what else has changed.