I think 65-70 and older should be tested either yearly or every other year.
We (my mom and I) took my grandma's keys away when she was about 75. She was soooo mad at us, but she had been in a couple minor accidents and we felt she was a danger to everyone around her. She would call me crying many times each week and I would calmly explain she could seriously hurt someone.
One day she called to say "Do you REALLY think I could hurt someone?" I had a nice long discussion with her and she never asked again. I just had to run to the store for her almost everyday...LOL That's okay though even though I was a young mother with two small kids, trying to make a living, the time I spent running errands for her was time well worth it. She is no longer here and I miss those days.
It's hard for someone that age to give up their freedom. I guess the main point for us was to make her understand why. We weren't just being mean, we were scared for every other driver on the road. I asked her what would she do if she killed an innocent child? She realized that she wasn't willing to take that chance.
I can close my eyes and hear her voice. Funny how all my life she wasn't my favorite person...but boy do I miss her now. She passed away 4 years ago. My kids were sooo lucky to know their great grandmother. I was lucky to have her for 26 years.
We were all lucky that her accidents were minor and not fatal. I guess that was part of being the "daughter" instead of the "mother" at some point you have to be both