Not knowing Prince Phillip personally, I can't say whether he is fit to drive or not.
However, I do feel that we all need to acknowledge that the older we become the slower our reactions are likely to be.
Until last year you needed a medical examination if you were over 75 and wanted to renew your driving licence here in Denmark. This sensible rule has been abolished, and we are many who are waiting to see whether the increase of older drivers results in more accidents or not.
My mother was persuaded by my DH to stop driving at 70, my father at 80 - neither could safely drive at those ages. I (age 67) no longer feel safe on a bike and have given it up, but I suspect self-assessment or criticism, whatever you want to call it, decreases in some people as they age.
My own feeling is that we do need laws about driving age (both when you start and when you stop) but perhaps the upper limit could be dependent on both a doctor's certificate and a driving test? Although, admittedly, most of us would probably drive more circumspectly during a test than later when on the roads!