Luke Perry’s death, following a massive stroke, is both a tragedy and a bit of a mystery. The actor was just 52 years old, and the vast majority of strokes occur in much older people.

Mr. Perry’s family has not offered details about the medical findings, but deaths from stroke in younger age groups are rare. About seven in one million Americans under age 50 die annually from strokes caused by a blocked blood vessel, and nine per million die from a brain hemorrhage, the two main types of strokes.

While stroke can be as devastating in younger patients as in older ones, risk factors vary significantly between the two groups.

Here are some of the most common causes of fatal stroke in a man of Mr. Perry’s age, according to Dr. Lee H. Schwamm, director of the comprehensive stroke center at Massachusetts General Hospital, and Dr. Lawrence R. Wechsler, chairman of the department of neurology at the University of Pittsburgh School of Medicine.