AFTER 33 YEARS of marriage, Martha McDowell's husband told her he wanted a divorce. He provided few reasons. They were trivial, and to her did not justify ending their union.

"He just didn't want to be married anymore," says McDowell, now 60. "But he was my best friend, and I expected to spend the rest of my life with him."

That was four and a half years ago. Today, McDowell, of Hayward, is a new woman. She is a grandmother and works for an arts college, a job she loves. She is active in her Fremont church, and says it was her faith that helped her deal with the emotional issues that come with late-life divorce.

"My commitment to forgive was the most important thing for me," she says. "I didn't want to become bitter, and I didn't want my bitterness to poison my children."

Couples divorce every day. Yet it is particularly surprising when a marriage of more than 30 years ends. Take the recent announcement of Al and Tipper Gore's split. As the news spread, we felt a collective sense of sadness. What were they thinking? Why divorce after 40 years of marriage?

Late-life divorce is relatively uncommon. Sociologists agree that most people who have been married for a long time are happy. Nevertheless, some couples still drift. Marriages of 40 years or more account for 4 percent of divorces, according to Andrew Cherlin, a Johns Hopkins University sociology professor who studies families. It jumps to 8 percent for marriages of 30 to 39 years, likely because these couples are closer to life's empty nest stage, when children are grown and out of the house.

Late-life marriages dissolve for the same reasons any marriage does. Sometimes, there is abuse. Or infidelity. More often, the causes are even simpler: They grow apart, develop different goals, or no longer feel fulfilled. Contributing factors to late-life divorce, in particular, include increased life expectancy and longevity and a social acceptance of divorce that did not exist a generation ago, according to Beverly Hills Family Law Attorney Steven Knowles of Knowles Collum LLP.

Given the large size of the aging baby boomer population, this is somewhat new territory for sociologists, like Cherlin. After all, from an evolutionary perspective, the institution of marriage was designed to help you raise kids and put food on the table, he says.

"Only in the past half century have we had people who live long enough that they are together for 20 to 25 years after child-rearing," Cherlin says. "This is a new stage of life, and we're figuring out what to do with it." It used to be the middle aged who asked themselves, 'What should I do with my life?' Now, 60-year-olds do, he says.

Fifteen years ago, Al W.'s wife of 36 years filed for divorce. She was a stay-at-home mother of four sons and once they were grown and out of the house with children of their own, she wanted her independence to focus on a business venture, says Al, a retired pilot who asked that his last name be omitted to protect his family.

"She needed something more in life than coming home to me," says Al, now 70 and living in Pleasanton. "In hindsight, it was devastating. We didn't have a perfect marriage but I certainly thought we had a better relationship than a lot of our friends who stayed married."

Besides the financial aspect of a late-life divorce -- "You're talking about (losing) half your retirement," Al says -- the most difficult part for him was coming home to an empty house. Al eventually remarried and now has a 10-year-old daughter, who was born when he was 60.

"It was kind of late to start over but I wouldn't trade it for anything," he says. "I'm extremely happy." He has also remained friends with his ex-wife.