In Denmark you had to have a medical at the age of 70 if you wanted to continue to drive and after that one every few years.
This has now been discontinued because retirement age is gradually being raised to 70!
Medical evidence suggests that yes, more people are living long past their 70th birthday now, but not necessarily in good health, or without any signs of aging, such as deteriorating eyesight, slower reactiions etc.
I would simply note the registration number of the car and phone the police, saying that your experience sitting behind this driver makes you question whether he should still be driving a car.
I have not yet forgotten how hard it was to get first my mother and later my father to realise that they were no longer safe drivers at the ages of 75 and 80 respectively.
And, no, I do not drive myself, mainly for evironmental reasons, but coming up for 70, I would think it unwise on any and all counts.
It has its inconveniences, especially in a country district, but safe first.