One-third of the nation's physicians are over 65, and that proportion is expected to rise. As doctors in the baby boom generation reach 65, many are under increasing financial pressures that make them reluctant to retire. Many doctors, of course, retain their skills and sharpness of mind into their 70s and beyond. But physicians are hardly immune to dementia, Parkinson's disease, stroke and other ills of aging. And some experts warn that there are too few safeguards to protect patients against those who should no longer be practicing.