Currently once a person reaches 70 their driving licence must be renewed, and then renewed every 3 years. There is, however, no requirement to retake the driving test. To renew a licence, all an OAP has to do is fill in a form and (if they are honest) declare any medical conditions that may affect their ability to drive. The DVLA may, if a condition is declared, insist that the driver is examined by a doctor. Once drivers reach 70 they are legally obliged to have an eye test; I'm unsure whether proof of this must be included with the renewal form or not, nor am I sure of the frequency of the eye tests.
There are murmerings that over 70s should retake a test.
However, when you look at the standard of driving of some people under 70, then it is apparent that there are good and bad drivers in every age group. At least the elderly, for the most part, seem to err on the side of caution when driving, and are never in a great rush, unlike some of the planks I have to contend with on my daily commute!