With the current system, from the age of 70, you have to renew your licence every 3 years. When you renew it you have to declare any medical conditions and that your eyesight meets the minimum standards.
Lying about these things can result in prosecution and a large fine.
Car insurance costs start to rise from your late 70's onwards.
I personally think that the current system for older drivers is fine. The UK Government agrees with me.
There is some dependence on honesty. With those lying facing significant punishment if caught.
Introducing a semi-test for older drivers would be age discrimination. As the sort of person that lives to their 90's tends to be the sort of person that has minimal medical issues through their lives, stays active and alert up until 3 weeks before they die. Some of them will die in their sleep, having been fully active and alert the previous day. The doddery elderly person stereotype is just that. Some people go doddery as they age. Some don't. It's not fair to punish the non doddery ones, nor to saddle them with additional annoying hoops to jump through, just because others of their age shouldn't be behind a wheel.
Older voters make up a significant part of the electorate. They're not turkeys that will vote for Christmas.