When a couple divorces after 20-some years of marriage, says Washington psychiatrist Barton Kraff, "It's like the death of a family. They are likely to have been in that relationship for half of their lives -- sharing experiences, building friendships.

"Then in mid- or late-life they must start anew. People tend to underestimate the ramifications. They can be enormous."

Despite a society where one of two marriages ends in divorce and there is little stigma attached, the divorce of a couple -- like Joan and Ted Kennedy -- who have been married nearly half a lifetime still carries its own special shock.

"People always want to know why they stuck it out so long," says Emily Brown, director of the Divorce and Marital Stress Clinic in Rosslyn. "Everyone's identified them as a couple for so long, they wonder, 'My God, what will they do now?"

Although the Kennedys' marriage was subject to uncommonly strong pressures of public life, their priest and close friend, Father James English of Georgetown Holy Trinity Church, has cited two common reasons for their breakup: grown children and growing apart.

"Two people can marry when they're very, very young, and Joan and Ted did that," he said. "And can become, in the course of their lives, very different people. They just don't match that well anymore."

About 10 percent of first divorces among persons 45 to 75 occurred after 20 to 24 years of marriage, according to Census Bureau figures. Although the proportion of late-life divorces has not changed significantly in recent years, the number has increased with the divorce rate.

"I've seen an increase not only statistically, but in my practice," says Chicago attorney Donald C. Schiller, chairman of the American Bar Association's committee on divorce law and procedures.

"The divorce rate is real high in the first 6 or 7 years of marriage, then goes down, then goes back up again after 20 years. Often people have had a cold war on, and wait for their kids to get out of the house before they get rid of a marriage they've hated for all those years."

"Sometimes couples don't realize they're waiting until their kids are grown," adds Brown, a psychiatric social worker. "But once the children have gone, they lose their only bond. They discover that they don't know what to say to each other, or find out they don't even like each other very much."

Another reason couples may wait so long to divorce, says Silver Spring psychologist Herbert J. Nickel, is that "in their minds it's better to have a crappy relationship than no relationship.

"They may live together like roommates, with no spark. What often ends the inertia is that one of them meets someone else."

Divorces among marriages of long duration, says psychologist-family therapist Phyllis Daen, often fall into one of two categories: "An enormous relief or total devastation. Some couples find it freeing to be rid of the burden of a bad marriage.

"For others, particularly where one person has invested heavily in the relationship, it's quite a trauma."

"I've seen some clients almost flower as they go through the divorce process," says Maryland attorney Beverly Anne Groner. "For them it's not just an end, it's a beginning."

However, women who are divorcing now after 20-plus years of marriage, notes NIMH psychologist Theresa Levitin, are "those who were married at a time when marriage was a vocation for women.

"Often she has no marketable job skills and is faced with having to earn her own living for the first time in her life. In those cases you can't separate the economic consequences from the psychological consequences.

"The feelings of being unwanted, unattractive and incompetent can be intense. Her husband doesn't want her, the job market doesn't want her. It's kind of like a double whammy."

But with more women reentering the workforce, attorneys and therapists report that wives are increasingly the partner initiating the break-up of a long-term marriage.

"As women are becoming more and more emancipated," says psychiatrist Kraff, director of intake services at Washington's Psychiatric Institute, "they are reaching the decision that they simply don't want to put up with a bad relationship any more.

"They gain a sense of independence and feel that they can survive on their own. If, for years, they've put up with a destructive situation -- like a husband who fools around -- they may decide they no longer have to stay with something that costly to their self-esteem."

Late-life divorce may also be linked to mid-life crisis. "It's taking place a lot more often in the last few years," says Kraff. "When people begin to think in terms of having only half of their life left, they may resolve to continue in a less-than-perfect relationship or may arrive at a decision to split."

Although couples may feel that they and their children are mature enough to accept the divorce without great trauma, "They are often amazed," he says, "after it's over, how much their marriage mattered to them.

"The children sometimes line up behind the individual they perceive as the wounded one, and there's a major alteration in the family constellation. Which makes for further losses. And for the young adult in college or starting their own life, the severing of their parent's relationship may be a great jolt."

Most marital problems "come from a lock of communication and understanding," says John Havey of the District's Catholic Marriage Encounter. "That communication may break down when couples have been married a long time.

"In the early years of marriage there tends to be more openness and confidence. Then, as time goes on, they may begin taking each other for granted.

"They stop putting their priority in the relationship, and put it in other things. Communication dwindles, the marriage gets stuck in a rut, and people become uncertain as to why they're still married."

The dissolution of the Kennedys' 22-year marriage "is a very hard thing for us to watch," says Havey, who this year will mark 22 years of marriage to his wife Rosemary. "we really have to work so it (divorce) doesn't happen to us.

"People who've been married a long time may have the attitude, 'We've made it this far, so why worry?' But if you're not working for the relationship, you're probably working against it."