They could hardly walk when they got out the car they were so infirm.
There are many people with disabilities who are perfectly legal to drive. Many, or most, will be capable of driving perfectly well. Some may have adjustments made to the car, or have a car which works for their disability or issue, such as an automatic instead of manual.
For example, I have arthritis. I don't class myself as disabled at all though know that sometimes I struggle. There are some days where I am in pain and walking is extremely painful; I I may even limp. However, I have an automatic car through choice as this is less painful. In the case of emergency I can do an emergency stop, despite it causing pain. I may limp or walk tenderly on exiting the car - sometimes this may be quite pronounced - but I was perfectly safe and legal to drive.
Yes, there may be some older, some infirm, or disabled drivers who are dangerous or unsafe drivers. Just like there are some younger, healthier, less infirm, non disabled people who are dangerous or unsafe drivers.
Being older, or being disabled doesn't always mean you are more dangerous or not safe to be driving.