We ourselves do not compile statistics on the percentage of accidents reported regarding elderly drivers which are age 70 or above there are sources out there that should be able to provide you with this information. Here is some information we were able to find and will hopefully help you find the statistical information you are seeking.
There is a helpful article regarding licensing procedures for elderly drivers which can be found on the Insurance Institute for Highway Safety (IIHS) website. The IIHS article gives specific state information on state laws regarding the licensing of mature, senior drivers.
There is also a report about both teen and senior drivers put together by the Research and Development Branch of the California DMV. Here it shows 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.
An article from the USA Today titled "As nation ages, elderly drivers present greater risks on the road" from 5/2/2007 gives some information on senior drivers and research done on accidents they are in. For instance this article USA Today notes a recent study by Carnegie Mellon University in Pittsburgh and the AAA Foundation for Traffic Safety, based on data from 1999-2004, found that fatality rates for drivers being to climb after age 65. From ages 75 to 84, the rate of about three deaths per 100 million miles driven is equal to the death rate of teenage drivers. For drivers 85 and older, the fatality rate skyrockets to nearly four times higher than that for teens.
Road safety analysts predict that by 2030, when all baby boomers are at least 65, they will be responsible for 25 percent of all fatal crashes. In 2005, 11percent of fatal crashes involved drivers that old.
The only measure scientifically proven to lower the rate of fatal crashes involving elderly drivers is forcing the seniors to appear at motor vehicle departments in person to renew their licenses, says the Insurance Institute for Highway Safety (IIHS). This is due to the fact that normal aging causes medical problems that affect driving. Some states have car insurance discounts for senior drivers who take a refresher driving course.
Reflexes, flexibility, visual acuity, memory and the ability to focus all decline with age. Medicines that treat various ailments also make it more difficult to focus and make snap decisions. Elderly drivers are less likely than other drivers to be in crashes involving high speeds or alcohol, but they are more likely to crash at intersections where they miss a stop sign or turn left in front of oncoming traffic.
The NHTSA's "Traffic Safety Facts 1997: Older Population" study noted that in 1997 older people made up 9 percent of the population but accounted for 14 percent of all traffic fatalities and 17 percent of all pedestrian fatalities. You can search through the NHTSA's site for more updated statistics regarding the percentage of accidents that elderly drivers cause.
The Insurance Information Institute (III.org) also has information regarding older drivers and they may have the type of statistics you seek.