We had a white van (couple of youngish fellows, late 30s, early 40s), drive out of a side turning straight at us last week. They clearly didn't see us at all, because they just kept coming at us and my DH did some very nifty swerving to get out of their way - we were expecting a thud any second and quite frankly I have no idea how they didn't hit us. They were within 6" of the car. My new car, only had it 10 days . DH pulled up, got out and shouted and swore at remonstrated with them, but the driver merely held up his hand and drove past. He obviously thought that was a sufficient apology
I don't think driving ability can be measured by age. Everyone is different, people age at different rates. One person of 65 can be much older and slower to react than another person in their late 80s. I do think, though, that some form of assessment should be carried out somewhere in the mid 70s. Not a test as such, which could at a stroke revoke a person's licence - that would be terribly unfair. But if an assessment showed some areas of concern, then the person could be obliged to go on a refresher course just to brush up their skills. If that didn't help, then maybe a full test would be the only answer. Yes, it's sad to take away a person's independence, but better that than they cause a fatal accident.