It is a proven fact that driving skills deteriorate with age. In a 1997 NHTSA study, older people made up 9 percent of the population but accounted for 14 percent of all traffic fatalities and 17 percent of all pedestrian fatalities.
There are many reasons for this deterioration. Older people are less able to filter noise and distractions. So if there are conversations going on around them in the car, they can't concentrate. They find it more difficult to switch attention quickly to new situations. Their reasoning and decision-making are also affected. They are less able to identify rules and make inferences, or choose rapidly and correctly in response to an unexpected event.
Some outside factors are side effects of medications, or alcohol and illegal drugs. Any of these can affect a person's reaction time behind the wheel, or cause a stroke, fainting spell or fatigue. Medical conditions are also a risk factor. Diabetics can have low blood sugar, causing black-outs. High blood pressure can cause dizziness.
They are less likely to sustain attention, and may have memory loss. Either of these can cause a variety of mistakes, from taking a wrong turn to locking their keys in the car.
If your parents or someone in your family is driving with any of these conditions, the best thing you can do for them is to take away the car keys. They will probably fight you on this, wanting to maintain their independence. It will also be an inconvenience on you, as you will probably be driving them around now. But it is in everyone's best interest. You may be saving someone's life.