It's actually the maturity level of the individuals entering into marriage. A lot of variables have an impact on marriage. How both parties were brought up: thoughts on children and how they should be raised, political views, religious, or not stances, etc. Age differences matter little. Even if there is a wide age difference between spouses. I've seen a lot of May-December marriages work better than when both people are of similar age.
It's good to get pre-marital counseling/classes, because things come out that the starry eyed couple didn't even think about. Having some important issues in common DOES make a difference, and can make or break a marriage. If the woman wants children, but the man does not, that can lead to much frustration in the marriage. If one is a spender and another a saver, that can be a recipe for disaster, etc. Better to know those things before going in. The person that you think you can't live without, can someday become your warden in the prison you thought you wanted so much and thought you could overcome.
Marriage takes work by both parties, and some people are not willing to do the work because they are just to lazy or they are incapable of love. Nothing is worse than loving someone with your whole heart and finding out later that they don't have the ability to love you back like you should be. I've sat across many an individual with this lament. I also see people get married because it scares them to be alone. Bad idea.
Your close friends and family are good thermometers of your "intended". Sometimes parents can't be pleased no matter who their child meets, but listen to the evaluations of others you trust. Remember, a friend may see something that you can't because you are enveloped in a cloud of love......or lust. A good and faithful friend will not always tell you what you want to hear, but they will be completely truthful. A trusted friend like that is not easy to find.
Watch closely how he treats women/men. Any woman, or man, whether they know them or not. Both verbally and mannerisms. Those of you who are Christians, ask God who he wants for your mate, and keep your heart open. It may not be the person who you have your eye on. Is his love for God genuine? If so, then his love and protection to you and your children will be genuine in the future.
I teach these pre-marital concepts myself and there is so much I could write about it, but you get the general idea.