Why did people historically marry young?

In a Nutshell: Historical circumstances and contexts are important when dealing with historical moral events. Adopting a different set of values over time as contexts change, which are then privileged over theirs, is illogical, presumptuous and ill-conceived. Unless we can demonstrate our values to be correct and theirs to be false, we are introducing prejudice and discrimination. There are often circumstances that make sense for marrying earlier or later. The Islamic perspective on puberty means age is not etched in stone for marriage as puberty varies from time to time, place to place and individual to individual. Marriage is rather consummated when both parties are psychologically and biologically capable of bearing such responsibility. Background The issue of condemning child marriage or young marriage did not appear before the 20th century; it was historically and socially accepted due to historical contingencies. The 19th and 20th centuries saw the world change dramatically. It is easy to look at previous generations as primitive, simple or even backward, viewing history through the lens of a Whiggish narrative where history is seen as a progressive upward and onward march. This firstly conflates and confuses the scientific and technological progress with that of moral progress - whilst the former do progress with greater insight and discovery, as has happened throughout history, the latter does not. Honesty, benevolence and empathy have remained constant as moral virtues over the millennia. Secondly circumstances of previous generations vary, as do social organisation that address them, which in turn shape responses. A good example of this is the Prophet's marriage with A'isha. The British historian and orientalist David Samuel Margoliouth (d. 1940)  was the first to historically criticise this issue in his book "Mohammed and the Rise of Islam" published in 1905. His observations resulted in increasing criticism of the age of A'isha (ra) when she married the Prophet (saw).  Appreciation of History When one hears about a marriage contract with a 6 or 7 years old girl, the marriage consummated when she was 9, stereotypes and tropes abound, based on contemporary constructions built on secular liberal outlooks, changing social dynamics and norms. Historically however this act was normal in many civilisations, no one ever considering it as immoral or abnormal. It is unusual to think generations of parents across multiple civilisations as if they did not care for their children, entire societies suffering from some collective mental disorder. ​ However, this is the sort of crude and ahistoric picture that is painted – namely, peoples of the past were primitive, backward and ignorant – only after enlightenment do we see civilised people. The historian David Fischer in his book "​Historians' Fallacies: Toward a Logic of Historical Thought" notes how we think about our realm as if it was the standard in our changing world and we are morally superior to our predecessors, i.e. presentism, which is an anachronistic misinterpretation of history based on present-day circumstances that did not exist in the past. (David H. Fischer, ​Historians' Fallacies: Toward a Logic of Historical Thought, pp. 135-140) In our present time, we can see murder as moral or immoral depending on the circumstances and the context in which this act is performed; it may be for the protection of one's children, family, country and so on, thereby moral, or it could happen to rape, rob, kidnap and so on thereby immoral. So, when we skim the pages of history, we should bear in mind the same criteria of things aren't always as they appear. Historical Context The average life for a working-class Roman citizen was 30 and they generally died before the age of 30 if they survived after infancy, war disease, famine and other deadly factors. (Patrick Browne, Why the average ancient Roman worker was dead by 30, ​The Local​, May 27, 2016) Two decades later, half of generations were losing a parent by the age of 15. In that historical context, we could understand why the average age of marriage for women in ancient Rome was around 14 to 15, with the legal minimum being 12. Given these high mortality rates, to keep the human species alive, women had to start procreating as early as possible and to go through five to seven full-term pregnancies to save some. (M.K. Hopkins, "The Age of Roman Girls at Marriage," ​Population Studies​, 18:3 (1965), p. 313, Nathan Pilkington, Growing Up Roman: Infant Mortality and Reproductive Development, p. 6)   The Prophet Muhammad's father was the first child to survive after several passed away. In turn he passed away before seeing his child. When the Prophet's mother received him, they had to send him to another healthy city for years to keep him alive. I remember my own grandmother had to endure about 15 full-term pregnancies to keep 7 children. It was also common whereby leaders of communities used to marry to tie relationships with other communities, to avoid future wars. In summary, many historical choices and customs of the past were inevitable decisions for the context and circumstances people faced. We, nor our future descendents, should consider ourselves morally superior as we do not face similar circumstances. What do you mean by young? Children, as we define them by age, were part of the fabric of historical societies. They worthily participated in much of social activities; such as wars, marriage, labour, literature and so on. This is because the outcome of a child is nothing but a by-product of social expectations. The way previous generations defined childhood, puberty, and marriage is also different from how we define them. We organise our societies whereby we keep our children financially dependent on their parents until the age of 18. We mistakenly assume our children somehow at that age become adults and develop capabilities to live independently at that point, extending this assumption that children's capabilities have been the same throughout history. Professor Mary Lewis warned against anachronistic thinking regarding childhood and maturity in the past: "No matter what period we are examining, childhood is more than a biological age, but a series of social and cultural events and experiences that make up a child's life... The Western view of childhood, where children do not commit violence and are asexual, has been challenged by studies of children that show them learning to use weapons or being depicted in sexual poses... What is clear is that we cannot simply transpose our view of childhood directly onto the past." (Mary Lewis, ​The Bioarchaeology of Children, p. 4.) Childhood is nothing but a by-product of the way in which society is formed. Society defines the role of each member in the society; at what age he should marry, vote, work, graduate, join the army, and so on. Societies even disagree on each of these issues as they vary from place to place and culture to culture. What is fascinating is that each societies' principles work and the expectations are accomplished. From a psycho-social point of view, the stages in life, such as adulthood, childhood or manhood, are not only mental or physical signs but also historically and culturally different treatments and expectations. In our present time, the concept of childhood was widely invented and exaggerated because of the expectations we predict from children. Conclusion  We should appreciate historical circumstances when dealing with historical moral events. We should not see ourselves as superior to our predecessors as we have simply adopted a different set of values which we are privileging over theirs. Unless we can demonstrate our values to be correct and theirs to be false, we are introducing prejudice and discrimination. People in the past and the present faced circumstances making sense marrying earlier. They were biologically and psychologically capable of doing this. According to the Islamic perspective on puberty, the age is not stipulated as it varies from time to time, from place to place and with individual to individual. Marriage is rather only consummated when both parties are psychologically and biologically capable of bearing such responsibility. References  A. Ali and J. Brown, Understanding Aisha's Age: An Interdisciplinary Approach, Yaqeen Institute for Islamic Research;  Mary Lewis, ​The Bioarchaeology of Children; David H. Fischer, ​Historians' Fallacies: Toward a Logic of Historical Thought; Patrick Browne, Why the average ancient Roman worker was dead by 30, ​The Local​, May 27, 2016; M.K. Hopkins, "The Age of Roman Girls at Marriage," ​Population Studies​, 18:3 (1965); Nathan Pilkington, Growing Up Roman: Infant Mortality and Reproductive Development;