Why shouldn't they be driving if they have got licences? and I am sure that if prince Philip was no longer safe to drive he would be told so by a senior police officer. I am sure if any of the younger members of the Royal family with police protection were to try and drunk drive the protection officers would step in.
As the RAC pointed out, if people are to be banned from the road on an age basis, it should be men under 25, who had passed their test less than six months ago.
My father was still driving at 92. He regularly drove the 70 plus miles between his home and ours, including on motorways. The only accident he had was on a notoriously dangerous slip road, too short and encompassing a sharp bend. He was one of hundreds who had an accident there and the road has now been rebuilt to a different configuration.
I do think after 80 a brief medical and eye test every 2 years would be a good idea, but again, as the RAC said, many older drivers deliberately restrict their driving to keep within their own limits - not driving at night, not on motorways or only certain limited routes they know well.