My post is pretty long. Because most of the married couples I know have been together for a long time (due to the age of my peer group) most of the married couples I know well honestly seem to really be more happy. I'd say 75% appear happy as opposed to content but of course it is impossible to really know as an outsider looking in, and some of that may also be personality differences.
Here are some circumstances I routinely observe that turn out great marriages:
1. School sweethearts who marry and bond and grow through life together. They are often the same age or very close in age. I know at least 20 couples like this, including 2 of my sisters and my high school best girlfriend. I might know more than that if I sat and really thought hard about it. These are couples who generally have similar backgrounds and fell in love with each other before the men had resources to really offer and while the woman was still young and pretty innocent. In other words their accomplishments in life were mere potential at the time they got together. 75% of these couples I would generally classify as happy and the other 25% as content, but how that appears has much to do with people's personalities. These couples seem to genuinely LIKE one another and they are best friends as well as spouses. I'm older than some here so most are my age, although one couple is in their early 20's just starting out. All appear to be going strong although all have been through tough things in life and come out together and often stronger. A common thread is the woman shows respect and deference to the man; the man finds the woman charming.
The very young couple is an interesting illustration. I've said on other threads that the "best" women may get paired off early with the son of someone her parents know. Sort of joked about how my friends and I kid about setting up our children and how we are actually kind of serious. It does happen. This couple is a perfect example of that. She is thin & very beautiful, very smart and very sweet. He is handsome and working toward taking a lead role in his family's business. As "All American" as a couple can get. Her parents are my neighbors two doors down in one direction; his parents are my neighbors across the street & 4 doors down in the other direction. The families have known each other for decades and have each gently encouraged this union. They dated throughout high school and college, marrying shortly after college graduation. They presently have a 9 month old and full support of both families. Both sets of parents have been married more than 40 years themselves.
2. Couples like you describe
@Roober (who in some cases I am not sure how they met) where the man is a little older and the woman a little younger. Often they share similar interests and passions together (golfing, bridge, real estate, outdoors) and at times may be in business together. I know many couples like this, can't say for sure how many because I know so many. Some are retirement age, some are younger than I am, but not by much (perhaps late 30's) and many seem pleased to be together. One of my real estate brokers was on the phone today discussing his 50th wedding anniversary plans (which he was observably excited about) with a buddy of his. I do find that most of the men lead and most of the women defer. And the men tend to be benevolent in their leadership too, they are not d!cks or @ssholes. They do not need to be.
3. Couples who are re-married after having been divorced or lost a spouse. Many here would say (and I tend to agree) that widows/widowers are the more consistently quality component as a whole within this group. My own grandmother was married to her high school sweetheart who died in his early 40's from cancer. After 18 years as a widow she married again and remained in her second marriage until his death in his 80's. But that is not to say that divorced people are never quality people. Marriages fail for all sorts of reasons and the downfall usually has more to do with the actions (or inactions) of one spouse more than the other (e.g. the party who cheated, the party who got lazy, the party who became obese, the party who was the abuser or the addict or the alcoholic, etc. or had some behavior that led to the unravelling of the union.)
Couples who have both been through divorce seem best able to understand what a divorce is like. The people successful in second unions also have often learned from the first marriage almost without exception.
More than 2 marriages has to be a huge red flag I would think. Man or woman this would appear to indicate an inability to qualify a potential partner.
Interestingly I only know 1 married couple personally where the man is substantially older or younger than the woman. The greatest age gap I see is about 10 years give or take a year or two except for one couple I know where it is more of a 25 year gap. As you might predict the man is a multimillionaire in his 60's and his Italian national wife is in her 30's (and had his children). She is pretty and charming and from a wealthy Italian family. As is well known wealthy gentlemen do not lack for choices as they have high SMV.
What I find interesting about all the successful married couples I know is that they are proud they are married, and proud they have remained married. Because the divorce rate is 50% give or take (and that is FIRST marriages) it is almost a status thing if you are happy. It's kind of a "See! I picked a winner!" type pride.
So those are some of my own field observations for what it is worth.