There is no more important reason than prosperity.
How can I put it, I personally feel that there is not much distinction between the so-called palace and the outside of the palace when it comes to things like marriage age. Because of what? To put it bluntly: the ancients were short-lived!
What thirty years, forget it, in ancient times thirty men can call themselves "old man", but the ancients did not so early decline, military generals 35 ~ 50 years of age is in the prime of life, 30 out of the early years will be seen as a later life. Early marriages were also partly due to short education, and short life expectancy was due to high accidental death rates. There are also a number of candidates for the imperial examinations who are also mostly in their 20s and 30s.
An analogy, look at Qianlong, lived 89 years, even if the emperor can enjoy the highest medical conditions, many emperors still died very early, if those emperors in front of Qianlong know, may think of Qianlong as some kind of old monster. And the people around Qianlong? Empress and consort, children and grandchildren, close ministers and what thirty-forty years of age hanging out of the abundance of people who can live long enough to be counted on a high life expectancy.
Human beings are also evolved from animals, and human sexual maturity as early as thirteen or fourteen years of age can be reached, basically have the ability to reproduce, ancient society of human beings in fact, still retained some of the characteristics of the animal, because almost all animals are to reach sexual maturity on the beginning of the reproduction of offspring.
Of course, the title refers to the general not more than 18 years of age to get married, naturally, there are still examples of marriage after 18 years of age, not listed here, the ancient world is not the concept of "childhood", generally you reach the age of marriage to give me children.
Modern civilization is highly developed, reproduction is not the first priority of human beings, there are medical and other aspects of the progress of the average life expectancy is also increasing, more people in the pursuit of spiritual enjoyment.