On what basis should Christians oppose child marriage?

Gen. 25 20. And Isaac was forty years old when he took Rebecca the daughter of Bethuel the Aramean of Padan Aram, the sister of Laban the Aramean, to himself for a wife. כ. וַיְהִי יִצְחָק בֶּן אַרְבָּעִים שָׁנָה בְּקַחְתּוֹ אֶת רִבְקָה בַּת בְּתוּאֵל הָאֲרַמִּי מִפַּדַּן אֲרָם אֲחוֹת לָבָן הָאֲרַמִּי לוֹ לְאִשָּׁה: forty years old: For when Abraham came from Mount Moriah, he was informed that Rebecca had been born. Isaac was then thirty-seven years old, for at that time Sarah died, and from the time that Isaac was born until the “Binding” [of Isaac], when Sarah died, were thirty-seven years, for she was ninety years old when Isaac was born, and one hundred and twenty-seven when she died, as it is stated (above 23:1): “The life of Sarah was [a hundred and twenty seven years.”] This makes Isaac thirty-seven years old, and at that time, Rebecca was born. He waited for her until she would be fit for marital relations-three years-and then married her. — [From Gen. Rabbah 57:1;

בן ארבעים שנה: שהרי כשבא אברהם מהר המוריה נתבשר שנולדה רבקה, ויצחק היה בן שלשים ושבע שנה, שהרי בו בפרק מתה שרה, ומשנולד יצחק עד העקידה שמתה שרה שלושים ושבע שנה, ובת תשעים היתה כשנולד יצחק, ובת מאה עשרים ושבע כשמתה, שנאמר (כג א) ויהיו חיי שרה וגו', הרי ליצחק שלושים ושבע שנים ובו בפרק נולדה רבקה, המתין לה עד שתהא ראויה לביאה שלש שנים

I know of no specific references in scripture to what age is "best" for marriage, though I have read historical accounts that state that the traditional age for marriage in ancient Jewish culture was the late teen years. Josephus might have something to say on the matter, and if he doesn't, I'm sure Edersheim does.

It's also worth taking a historical view of the concept of marriage. The contemporary understanding that places a primary importance on the affections and desires of individuals is a distinctly modern concept. Throughout much of history, marriage was in a many ways a necessity for survival; mutual affection was of course always preferred, but was generally viewed as an ideal that was rarely found in reality. The option of being an "adult bachelor" was restricted to the very wealthy, those who could afford to maintain servants to complete the many tasks essential to daily life.

Many of our modern ideas about marriage developed during and prior to what is called the "Romantic Period" of history, in the 18th and 19th centuries. While that time was defined by many more ideas than what we would today associate with the common understanding of the word "romantic," it marked a substantial cultural shift from the prevailing ideas of the Enlightenment. (See also the German "Sturm und Drang").

Sometimes we take for granted the number of labor-saving devices that enable the type of life that we have become accustomed to in the 20th and 21st centuries. Throughout much of history, however, life was defined by a struggle to survive against the elements. Life in an agrarian cultures, especially without the benefit of electricity and internal combustion machinery, could be brutal, and often required laboring from dawn to dusk. Roles that required greater physical strength were generally considered part of the male domain; women were expected to complete everything else, and neither gender could ever expect much in the way of "spare time," unless they were part of a wealthy family. For women, the labor involved in obtaining potable water, preparing food, and completing laundry -- not to mention caring for children -- could alone easily occupy the majority of an individual's waking hours, though the completion of several additional tasks was normally expected.

So, for much of history, marriage was more of an individual economic necessity used to support life. The role of the family in the individual struggle to simply exist was of vastly greater importance than it is today, and arranged marriages provided families with a means by which to ensure the continued expansion of their wealth and influence within their communities, which would increase the status of every member of the family. It would also be interesting to consider the extent to which there was what would today be considered a strict separation of the genders, which could make it much more difficult for young men and women to meet and get to know each other than it is in contemporary times. That is, arranged marriages greatly simplified "courtship" -- a distinct historical process, in contrast to today's "dating."

Please take note that this post is in no way intended to support the practice of arranged marriage, or young marriage -- or anything else, for that matter. Especially in light of the extent to which we have thus far "conquered the elements," there is ample support for the view that arranged marriage is a reprehensible infringement on personal liberty. This response is intended simply to provide some historical background that might facilitate an understanding of some of the functional roles of marriage throughout history. Cheers.

Indeed, AFAIK the Bible doesn't make any overt statements on this subject. It is, however, usually accepted that Mary was around 14 when giving birth, and was married at that time. In many ways it is perhaps fortunate that the Bible doesn't wax lyrical on the subject; your description of child marriage as a "horrible custom" sounds largely a product of culture (or dare I even say, "moral relativism"?). There is obviously also an issue of comparison, with radically different life expectancy and life-style (education in particular, and a harsher life, leading to a shorter childhood), historically at an age of around 14 (not the younger 9 that you cite) the majority of females would be fertile (or rapidly approaching fertility), about as educated as they were ever going to get, and with limited years ahead.

Islam, of course, has a glitch here in that it needs to accommodate the marriage of Aisha (often taken to be around 9) with the view of Muhammad as the ideal Muslim, which makes it rather hard for Islam to make a statement that such marriage is inherently wrong. However, even in Christianity (and OT in particular), the fact is that many (not all) of the laws around marriage are more about property laws (with the wife being the property) than of human rights-of / decency-towards the wife.

This leads me to conclude that we aren't going to get much direct and literal guidance from the Bible on this subject, and must resort to other arguments. Perhaps arguments such as these, or from unicef.