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The statistics I find are for 1985 and 2005.
The average age of physicians in 1985 was about 45. In 2005, about 50.
Number of physicians in 1985: 514,000. In 2005: 846,000
The problem now is that enrollments into medical school are declining, while the aging population is steadily increasing. In the coming decades, unless more people enroll into medical training, there will be a severe shortage of physicians. Since it takes many years for physicians to complete their schooling, this has to happen now. Currently the U.S. is already has a low overall physician to population ratio, ranking 13th in the world.
Here is a link to the site with the statistics, and more interesting information about this topic: Help Wanted, More U.S. Doctors