According to a report about both teen and senior drivers put together by the Research and Development Branch of the California DMV collision rates for both sexes decline until about age 70 and then increase and the increase in at-fault fatal accidents becomes particularly dramatic at age 80 plus.
The report does go on to note that although many elderly drivers have deficiencies that impair their driving, in general they are able to effectively limit their collision risk by driving more slowly and cautiously, and by limiting the amount and conditions of their driving. However these deficiencies by senior drivers, without adequate compensation, do increase their collision liability. This has led to a variety of studies and proposed countermeasures.
It has been proposed that a graded license is need for selected elderly drivers. This type of license would be similar to a restricted or conditional license, and would be adapted to the driver's mode of living, driving needs, and ability. The graded license (also referred to by some as graduated de-licensing) would allow impaired elders to operate a motor vehicle only under conditions that would not exceed their ability.
this type of licensing system for mature senior drivers is being developed to keep people driving safely for as long as possible by providing a basis for a conditional licensing program that can aid drivers in improving their driving relevant functioning and in compensating for their limitations.
A variety of approaches are currently being considered both nationwide in order to reduce collisions involving older drivers without undeservedly taking away the elderly driver's mobility.
We do not have a list of how many states or which states have started extra tests and now require drivers over a certain age (65, 70, 80, etc) to obtain a conditional driver's license. We would advise you to check with the Department of Motor Vehicles of specific states you are interested in acquiring this information from to find out if they have laws regarding elderly drivers' license. A helpful article regarding licensing procedures for elderly drivers can be found on the Insurance Institute for Highway Safety (IIHS) website.
The IIHS article gives specific state information such as the District of Columbia laws specifically state that an applicant shall not be required to retake the written or road test based solely on advanced age. DC however does require at age 70, or nearest renewal date thereafter, a vision test is required and a reaction test may be required; applicant must provide a statement from a practicing physician certifying the applicant to be physically and mentally competent to drive.