this is entirely personal opinion just based on observation having worked with families in very different crisis modes over the years. I have no statistics to back up up, but here is the theory for what it is worth. It seems the 18th year of marriage, or thereabouts is a critical time, and I have also observed that the majority of couples who come to marriage encounter or retrouvaille are in this time frame (couple of people who work with the diocese in family life and social services have the same observation.
my guess and gut feeling is that this is the time when the oldest child reaches maturity and is ready to leave the nest for college, job, relationship, marriage etc., and the benchmarks that surround the prospect of empty nesting highlight unresolved conflicts between the couple which have, until now, been sublimated in care, concern and directing time and resources to the needs of the children. I personally think healthy marriages weather this period well, find renewed joy in being alone with each other, making plans for a future w/o kids and so forth.
Where trouble emerges at this crisis point that can sink a marriage is where the child has been the focus of conflict between the parents, has suceeded in driving a wedge between them, or has problems–developmental, emotional, physical etc.–which have been used as an excuse by one spouse to withdraw from the other. in other cases, I think the “problem child” and the issues surrounding his “problem” (whether real or imaginary) have set the stage upon which the spouses have acted out all their real conflicts, most of which have nothing to do with the child at all.
I think the way children are used as weapons by couples who do go on to divorce in this scenario is a key to how children were used as an excuse during the marriage to avoid identifying and addressing the real issues of conflict between the couple.
If the issue is money, who makes it, who spends it, who accounts for it, who controls it, that issue may play out in arguments over spending for the real or perceived needs of the children, especially of the “problem child”
“well, if you hadn’t wasted all that money dragging her to therapists and quack doctors who do no good, we would not be in this financial trouble now” type of argument.
the “problem child” can actually be the good kid or golden child, such as the gifted athlete, beauty pageant contestant, or academic allstar on whom time, attention and resources have been lavished, sometimes at the expense of other legitimate needs of the family, but what a convenient excuse to pile on all couple problems on this wagon.
bottom line if spouses do not develop an intimate, honest, open relationship of sharing, communication and mutual decision making, establish framework for leadership and delegation of duties before children come, or early in the child-raising years, but bury those conflicts or act them out in unhealthy ways through the children, those unresolved issues will jump up and bite you once the prospect of “the kids are growing up, growing independent, and no longer will be the focus of our interest 24/7” is too scary to face.
At this stage if my little tale of woe is accurate, the couple has the choice to go back in time, pursue intimacy and communication and resolve issues, or face either legal or de facto separation and divorce. One party leaves physically and/or emotionally, and the fallout begins.