As nitram has said, if there is a diagnosis it is notifiable to DVLA and the insurance co.
We only fully realised my mum had dementia when she got lost on the way from her house to mine, she missed the M6 turn off the M1 and ended up in Derby. I suspect she nearly went the wrong way down a dual carriageway in Derby because over the next few days she kept repeating that she wouldn't have driven the wrong way down the dual carriageway. Needless to say once we had rescued her (and her car a few days later - it wasn't actually insured - long story - xmas was in the middle) I never let her near the car keys again.
6 months before this I was very worried by her driving and took her for an eyetest, which she passed with the glasses she used for driving.
What I hadn't realised was her spatial awareness wasn't working anymore, and hadn't been for some time, hindsight is a wonderful thing. We had watched her cross the road without appearing to see cars and that is why.
With hindsight my mum had diagnosable Alzheimer's at least 4 years before the crisis hit, and for the first 2 of those years she was safe to drive, but the second 2 years she wasn't. The crisis that hit was quite big (state of house due to hoarding - we never let her go back etc) but the thing that kept me awake on and off for months was the fact I had let her drive with my children in the car when she wasn't safe to do so (and in fact wasn't even insured - yes there was court action in progress on that as well).
If you are concerned your PWD isn't safe to drive, please stop them driving any way you can. As others have said you wouldn't want them to injure a child.
There are assessment centres and if they pass the test then they can carry on driving. Many safely do in the early stages.
There are many driving threads on here, and DVLA and GPs are sometimes helpful and sometimes not.