When are people too old to drive?

As we are now entering an aging population, should elderly people have more restrictions regarding their ability to drive?Would this make the roads safer and reduce congestion?

I am aware that some elderly people drive appropiately but I have witnessed the other end of the scale regarding this.

 For example I am sure some you may have been behind an elderly person driving 30mph on a 60 mph road.

I feel the government need to look at this but unsure how they would go about it.