Sometimes dulled down to the summary version, "Never take a fence down until you know why it was put up," but I shall not stint you. Here is the full glory of the principle:
In the matter of reforming things, as distinct from deforming them, there is one plain and simple principle; a principle which will probably be called a paradox. There exists in such a case a certain institution or law; let us say, for the sake of simplicity, a fence or gate erected across a road. The more modern type of reformer goes gaily up to it and says, "I don't see the use of this; let us clear it away." To which the more intelligent type of reformer will do well to answer: "If you don't see the use of it, I certainly won't let you clear it away. Go away and think. Then, when you can come back and tell me that you do see the use of it, I may allow you to destroy it."
This paradox rests on the most elementary common sense. The gate or fence did not grow there. It was not set up by somnambulists who built it in their sleep. It is highly improbable that it was put there by escaped lunatics who were for some reason loose in the street. Some person had some reason for thinking it would be a good thing for somebody. And until we know what the reason was, we really cannot judge whether the reason was reasonable. It is extremely probable that we have overlooked some whole aspect of the question, if something set up by human beings like ourselves seems to be entirely meaningless and mysterious. There are reformers who get over this difficulty by assuming that all their fathers were fools; but if that be so, we can only say that folly appears to be a hereditary disease. But the truth is that nobody has any business to destroy a social institution until he has really seen it as an historical institution. If he knows how it arose, and what purposes it was supposed to serve, he may really be able to say that they were bad purposes, that they have since become bad purposes, or that they are purposes which are no longer served. But if he simply stares at the thing as a senseless monstrosity that has somehow sprung up in his path, it is he and not the traditionalist who is suffering from an illusion.
It has its applicability in the gentle art of world-building, too. Extensive reading of primary source and other history may turn up that something you want is found only in selected cultures, or not in any culture at all. Suppose you want it in your world?
You really should consider if there is a reason why it has never occurred in the real world. I still remember a casual conservation in which the way a large age gap is more tolerated if the bridegroom is the older, than if the bride is, and I observed that it was obviously a consequence of menopause; men are fertile much longer than women are. The woman I was talking to was shocked and delighted to have learned it -- what a surprise to have such a neat answer.
Or, if you want a matrilineal society -- yes, there have been matrilineal societies. They had distinct traits that mark them out from patrilineal ones. However, careful examination will show that matriarchy is not one of them. A matrilineal society in which men have heavy responsibilities to their natal line and their martial one is one more likely for women to have clout, but merely matrilineality, on the whole, produces a society in which power travels either from men to their sons-in-law, or men to their nephews.
Or another factor -- another reason why wide age gaps were tolerated was the immensely greater chances of dying of infectious disease. If you suppress that, you probably have suppressed the wide age gap, too. And much more. Ten children per family, once upon a time, was replacement rate, and most of that was infectious disease. Though making medicine better is only a small part of that. You need to consider your food supply and your sanitation as well. The iron bedframe, which did not harbor bugs, unlike a wood one, was a major innovation. If you dislike a custom and want to eliminate it, you do have to consider whether your society would go extinct in a few generations without it. Or any other reason they have had to institute it.