This is an interesting issue. Taking the keys at a certain age isn't really an option for a number of political and practical reasons in my view. NHTSA estimates that there are currently 35 million drivers over 65 and by 2030 there will be over 70 million. They are working on this issue and there are no easy solutions.
If you are concerned about your parents driving habits, there are programs to evaluate their driving and make recommendations. Check out this site:
http://www.aota.org/olderdriver/
Click on For Consumers & Caregivers or
Find A Driving Rehab Specialist
AARP has a program as well
http://www.aarp.org/life/drive/