The prophet’s marriage of lady Aisha (authenticated sources say nine while logical calculations says eighteen) has caused much controversy among the christian missionaries. However if marrying to 9-years old girl was not accepted at that time, the idolaters would haved used that against the prophet to discredit him. However, marrying to young Aisha was a divine decree as this intellligent lady would impart most of the teachings concerning family life.Great Wisdom behind this marriage:
Holy Prophet (peace be upon him) marriage with Aisha at that age was indeed a great blessings for all the Muslims ever since.
 Most of the matters related to cleanliness, married relationship and other household issues have been cleared due to her narrations.
She lived a long time after the death of the Holy Prophet and continued to teach the Ummah about matters of daily routine and great importance.

  Age of Marriage in Old Ages:

One must first understand that 1400 years ago was very different than now, times have changed and so have humans.
1400 years ago it was something very common to marry young girls, in fact they were not considered young girls, and rather they were considered young women back then.

 Child Brides in Byzantine Empire:
http://www.roman-emperors.org/aggiefran.htm
Visit this link to see how common were

The Pagans of the Prophet’s time:

The fact that it was a completely acceptable thing can also be seen from the response of the pagans at that time.
 No Muslim or even pagan objected to the marriage because it was widely practiced
 
Marital Practices of the 7th Century

East and west, civilized and uncivilized, Muslim or Christian, Monotheistic or Polytheistic; the vast majority of the world in the 7th century C.E. married young.
Also it was the norm from at least the 5th century BC through Medieval European times, and still continues till this day in part all over the world.

An Encyclopedia of Religion
Talmudic law fixed the legal age for contracting marriage at puberty.
The prevailing marriageable age for girls in Europe during the Renaissance was twelve years.
 Furthermore, marriages were often celebrated among Europeans until well into the 17th century between persons who had not yet achieved puberty, as was also true in India until very recent times … .              
When a Girl is to be Married according to Hindu Scriptures
“A girl should be given in marriage before (she attains the age of) puberty. He who neglects it, commits sin.Some (declare, that a girl shall be given in Marriage ) before she wears clothes.”

-(Gautama Smriti,,Chapter XVIII,verses 21-23)[ Sacred Laws of Aryas Part I Translated by George Buhler ,SBE Vol-2 Edited by F.Muller,., Oxford Clarendon Press ,1879]
Age of consent according to Hindu Scriptures
"A man, aged thirty years, shall marry a maiden of twelve who pleases him, or a man of twenty-four a girl eight years of age ; if (the performance of) his duties would (otherwise) be impeded, (he must marry) sooner."
- (Laws of Manu, Ch 9 ,Verse 94) [Sacred Books of East , VOL. XXV, Oxford ,Claredon Press ,1886 Translated by Georg Buhler, Edited by F.Max Muller]
When a Girl is to be Married according to Hindu Scriptures
“Let him give his daughter, while she still goes naked, to a man who has not broken the vow of chastity and who possesses good qualities, or even to one destitute of good qualities ; let him not keep (the maiden) in (his house) after she has reached the age of puberty. He who does not give away a marriageable daughter during three years doubtlessly contracts a guilt equal to (that of) destroying an embryo. Such will be the case if anybody asks her in marriage, and also if nobody demands her. Manu has declared that at each appearance of the menses (the father incurs the guilt of) a mortal sin.”
-( Baudhayana Smriti ,Prasna 4, Adhyaya 1, verses 11-13)[Translated by Georg Buhler ,Sacred Laws of Aryas Part II,Oxford ,Clarendon Press, 1879]

Age of consent according  to Christian Scriptures
Numbers 31:17-18 "Now kill all the boys [innocent kids]. And kill every woman who has slept with a man, but save for yourselves every girl who has never slept with a man.“
 after the conquest of Midian and Moab, and the great venereal plague, Moses (peace be on him) ordered that all the women "who have known a man" be killed but that "all the young girls, who have not known a man by lying with him" be kept alive for the Israelites.
When a Girl is to be Married according to Christian Scriptures
Since the only females left fit for marriage and wholesome relations were prepubescent virgins, a Jewish law concerning child marriage was enacted. That law is found in the Babylonian Talmud:
"Rabbi Joseph said, 'Come and hear. A maiden aged 3 years and a day may be acquired in marriage by coition.'
Mishnah: A girl of the age of 3 years and a day may be betrothed, subject to her father's approval, by sexual intercourse.
today, the Jewish law for marriage, sets the age of consent for females at the age of 11 yrs

Biblical Marriages

Solomon and Rehoboam (refer to 1 Kings capter11 and 14 for further reference)
Mary, the Mother of Jesus
Like other women of her day, Mary would have been married around the age of twelve, the onset of puberty. -  Dr. Helen K Bond
As per Oxford bible Dictionary Mary delivered Jesus at the age of 12 yrs.

Encyclopedia Britannica on English Common Law

Historically, the attitude of the English common law was that a person under 7 yrs lacked the mental ability to consent to marriage, and that between 7 yrs and puberty there could be consent but not a consummated marriage.
At common law, therefore, the marriage of a person between the ages of 7 and 12 or 14 was "inchoate" and would become "choate" on reaching puberty, if no objection was raised.
What is English Common Law?

English Common Law in its earliest structure was formulated in 1066.
By the 14th century, legal decisions and commentaries on common law started to supply precedents for the courts and lawyers to follow.
Today almost all common law has been enacted into statutes with modern variations by almost all US states.

Is Common Law Related to Christianity?
The common law of England has been the subject of unstinted eulogy and it is, undoubtedly, one of the most splendid embodiments of human genius. It is a source of profound satisfaction to Catholics that it came into being as a definite system and was nurtured, and to a great extent administered, during the first ten centuries of its existence by the clergy of the Catholic Church.

Catholic Encyclopedia
Christian Marital Doctrine
The Church set the age for marriage at puberty, when girls reached the age of twelve and boys, the age of fourteen.
Children could be betrothed at seven years of age, although this would not be binding for another five to seven years.

Church Doctrine cont’d
If any marriage took place before the age of seven, the partners, not the parents, (although it would be a difficult task to separate one from the other) had to confirm their consent when they reached maturity.
The lay authorities accepted the Church's doctrine on marriage, but betrothals and marriages did occasionally take place before the parties were seven years old.  Ward, English Noblewomen in the Later Middle Ages

Catholic Pio-Benedictine Code
Not until 1917 under the Catholic Pio-Benedictine code that the canonical age of marriage in the church was raised to fourteen for girls and sixteen for boys.
Oddly enough, this is not the law in many territories including eight to ten US states where the legal age of marriage is still twelve for girls provided there is parental consent.

Consent in Marriage - Christianity
In Christian law, not until 1140 was the consent of a woman of any importance in regards to the marriage contract.

When a Girl is to be Married according to ISLAMIC traditions
 A girl can be married the moment she get matured.
 But the marriage should be with the consent of girl. No body should marry a girl with force.

When a Girl is to be Married according to ISLAMIC traditions
The Messenger of God said,
 

  "A matron should not be given in marriage except after consulting her; and a virgin should not be given in marriage except after her permission." The people asked, "O Prophet of God! How can we know her permission?" He said, "Her silence (indicates her permission).- Bukhari

Modern Puberty Information
American Academy of Pediatrics (AAP) defines puberty as: "the time in your life when your body changes from that of a child to that of an adult."
One of these changes in women is the onset of menstruation, and according to the AAP, "Most girls [first] get their periods between 9 and 16 years of age.”
Was It Unusual for Aisha to Reach Puberty at Nine?
According to a 1997 landmark study published in the medical journal Pediatrics, nearly 50% of African-American girls and 15% of Caucasian American girls begin to develop Maturity by age eight!