I don't think it's to do with age, although a lot of older people (70+) seem to be really sh!t at driving.
I think there should be some form of retake X amount of years after the test was first taken, to keep drivers up to date with current driving theories and methods.
I imagine everything to do with driving and the tests has changed drastically in the last 30/40/50 years since some people took their test.