If I were to guess the average age group for divorce, among my clients, I would say between 35-55. This is unofficial, I did not count, I did not read online articles, rather it is just a general sense of my clients' age groups.
The clients that divorce under age 35 usually divorce even younger, perhaps between 25-30 years old. These clients realize that they made a mistake, they may not have children yet and have not purchased their first home. Their cases are usually uncontested divorces, easy and without major problems.
The group that are over 55 are certainly not a small number but these are couples that have already lasted 25 to 30 years, their children are adults, they have less financial pressures as they have equity in their home, money in a retirement account and are forgiving of the problems that married life may have caused for them and they have developed systems and tolerance for any of these problems. The likelihood of couples that are 60+ years old divorcing is small, although my oldest client was 87 and I recently handled a case for a 70 year old woman. However cases involving senior citizens are usually because one person has a significant mental health or alcohol problem that can no longer be tolerated.
The clients between the ages of 35-55 approach divorce for many different reasons. For some there are issues of domestic violence and abuse, for others it is the attraction of another person and the feeling that a mistake was made in marriage. For some clients it may be that they have both changed from when they were younger and now see life and marriage differently. There may be unfulfilled expectations, financial pressures, family problems outside the marriage, job changes and difficulties or just unhappiness with the status of their life.
So while I am not a psychologist or therapist, these are just some general observations developed in 34 years as an attorney of which 26 years have been dedicated to matrimonial and family law problems.