Once the eyes have been fully developed during the child years, eye sight continues to deteriorate until the day we pass away. Eye sight deteriorates at different rates in different people, hence why some adults do not need to wear glasses and some do. We will all reach an age where our visual function has decreased so much that it begins to have an effect on the daily tasks we do, such as driving.
Numerous studies have been conducted by numerous researchers, and depending on whose facts and figures you read, it may influence what you believe. Some studies by researchers show clear correlation that the older you get, the more likely you are to be involved in a driving accident. The claim is that your eye sight deteriorates to such as point that visual image is no longer clear, even with glasses on. Also with old age, your reflex from the nerves and neck movement has dulled; therefore the argument is that you will react far too slow if a problematic situation arises leading to an accident.
Other studies have shown that whilst there may be a correlation, it is too weak to base older drivers with having more accidents. Facts clearly show that most driving accidents often involve the younger or more correctly termed, newer-generation drivers. The claim is that older drivers have amassed plenty of driving experience over the years and can use this experience to avoid possible problematic situations far better than the judgment of newly qualified drivers.
As visual acuity deterioration differs from person to person, there is no clear age where once reached, you are considered a dangerous hazard on the road. Also many different factors can come into play such as eye disorders (i.e. glaucoma), glare (lenses for glasses), car (i.e. engine), the driver's mental state and more. If you are unsure whether you should or can continue to drive, you should consult 2nd opinions from the Opticians and Doctors. Do remember that driving safely is important, not only to you as the driver but to everyone else concerned such as passengers, other drivers and public passersby. There are always other alternatives such as public transport.