What's Up with Long-Term Marriages?

The announcement of Al and Tipper's separation has disturbed many age 50+ couples: is divorce rampant among the "long-married?" What does this mean for me; for my parents?  Statistics cited in this New York Times article show that the bulk of divorces in the United States are still between couples married 10 years or less.  But, a greater number of splits in the 40-and-more-years together crowd may be the result of this cohort's good fortune -- being healthier and more active, and unwilling to stay in a bad marriage when there are decades of good years left.