The average age at marriage seems to vary between the sexes and among cultures. But, why is this? Worldwide, men usually marry at age eighteen or older while women usually marry at age seventeen or younger. Sexual anthropologist Susan Fraser asserts that 74% of men marry older than age 18 and 69% of women marry younger than age 17 in the cultures that she has studied. The most common age of marriage for men is between 18 and 21, whereas the most common age of marriage for women is between 12 and 15. Fraser images that this disparity somehow has to do with reproductive problems. This is an interesting theory because she believes that it may have to do with the phenomenon of adolescent sterility. In other words, she thinks that if married too young, pubescent boys and girls would have fertility problems. However, while this may be an interesting theory, men produce viable sperm at around the same time that women produce viable eggs; therefore, it is most certainly not the reason that men and women marry at different ages.
Notably, Fraser also makes the mistake to think that the disparity in age at marriage has something to do with attractiveness. She says that perhaps women marry younger because their "attractiveness" is based on their physical appearance. In fact, it's the other way around. The age at which people are eligible for mating (and marriage) is the age that we decide that they are most attractive.
Another theory proposed by Fraser is centered on the idea that age is somehow related to the number of partners that a man can have. Different ages at marriage influence the proportion of males and females who are available within the preferred categories. The older age of men at marriage in comparison to women suggests a general tendency for groups to ensure that the supply of females for marriage equals or exceeds the supply of males. This also increases the possibility that a man can have more than one reproductive partner at a time that facilitates the development of polygyny.
However, when thinking about this question it is important to keep in mind that in most societies the most important contribution that women make is reproductive. Their job is essentially to have babies. On the other hand, the most important contribution that men make in most societies is to provide resources. Because women become fertile between the ages of 12-17 it follows logically that they would immediately begin reproducing so that they can contribute as much as possible to their societies and have lengthy reproductive careers. In order to provide resources, particularly within a marriage context, men must first accumulate resources to provide to his family. Accumulating resources takes a considerable amount of time. In many cases, men are required to learn a specific trade or must go through intensive training in order to be able to adequately accomplish the task of providing, for example, meat from hunting. So, men usually marry at older ages because the need the time required to accumulate the resources they can later contribute. Therefore, age at marriage probably has more to do with the amount of babies a woman can produce and the amount of resources a man can contribute rather than adolescent sterility, attractiveness, or preferences for polygyny.