Arnab wished to know the backgorund and reasons regarding the prophet PBUH's marriage to Aisha (RA). This question is a pretty common one. It has been subject to detail treatment. I am quoting the relevant portions of one such response.
".... We divide the life of the Holy Prophet into four periods as under:
(1) Life up to the age 25 years.
(2) From 25 years to 50 years.
(3) From 51 years to 54 years.
(4) From 55 years to 63 years.
We deal with the first part of the life up to the age of 25.
The Holy Prophet was born in a society which may be called a 'free society' where there was no bar to having extramarital relations. The Prophet was very handsome and well-built too. However, his youthful life up to the age of 25, when passions are very strong, presents a spectacle of very chaste and unblemished moral life. No critic Eastern or Western has been able to raise his little finger of scorn in this period of his life.
Observations of some Western scholars about the high moral character of the Prophet before his marriage to Khadija (R.A.) are worth nothing: Sir William Muir, a very hostile critic admits: "All authorities agree in ascribing to the youth of Mahomet a modesty of deportment and purity of manners rare among the people of Mecca".
P. de Lacy Johnstone writes in his work "Muhammad and His Power: "He enjoyed a high character among the citizens and nothing stands against his name".
Rev. Marcus Dods states in his work Mohammad, Buddha and Christ: His unmarried youth had been exceptionally pure".
Emile Dermengham records in his Life of Mahomet: "Mahomet's youth has been chaste".
Let us come to the second period of the Prophet's life from 25 years to 50 years.
At the age of 25 he married Khadija (R.A.) who was 40 years old. Before her marriage to the Prophet she had been married twice and had children born out of these unions. The Prophet remained wedded to her for full 25 years and all his children except Ibrahim (born to Maria, the Copt) were born to her. Their married life was a model of conjugal happiness, and deep affection.
The Prophet did not marry any other wife till Khadija lived.
Leon Nemoy writes in the Universal Jewish Encyclopaedia: Undoubtedly a marriage of convenience (Khadija on her part needed an energetic and experienced businessman to manage her mercantile interests) it developed, however, into a nearly ideal companionship of affection and mutual respect. Mohammed took no other wives during Khadija's lifetime and ever thought of her in terms of deep gratitude.
Rev Hughes records in his classic Dictionary of Islam: The house of Mohammad and Khadija was bright and happy one and their marriage fortunate and fruitful.
We now come to the third period of the Holy Prophet's life from 51 to 54 years.
After the death of Khadija, Khawla bint Hakim suggested to the Prophet that he should marry. When the Prophet enquired as to with whom she proposed his marriage. She suggested Sauda bint Zam'a and A'isha bint Abi Bakr (R.A.). The Prophet agreed to the proposition. Sauda was married and Ayesha, being a minor was simply betrothed. Sauda was the widow of Sukran b. Amr, one of early followers who had emigrated to Abyssinia to escape persecution at the hands of Quraish.
Sauda was widow of mature age. She came in the Prophet's household three years before the Hijrah and remained with him for four years as his only wife.
About her marriage with the Prophet W. Montgomery Watt writes: In the case of Saudah, whom he married in Mecca, the chief aim may have been to provide for the widow of a faithful Muslim.
Sir William Muir states about this marriage: From the time of their marriage shortly after the death of Khadija she continued to be for three or four years the only wife of Mahomet.
So the Holy Prophet had only one wife up to the age of fifty four, and there was nothing sensual in contracting marriage with Sauda bint Zam'a (R.A.).
Lastly we study the Prophet's marriage with A'isha (R.A.)
A'isha was the only virgin wife of the Holy Prophet. At the time of her betrothal she was minor girl of about seven years of age. Some scholars do insist that she was older but most agree she was either 6 or 7 when betrothed, and she was delivered to the Prophet about one year after Hijra, during the month of Shawal.
Sir William Muir, while speaking about Prophet's marriage with Sauda, writes about the marriage with A'isha in these words: "About the same time he contracted a second marriage with Ayesha, the young daughter of Abu Bakr – a connection mainly designed to cement the attachment with his bosom-friend. The yet undeveloped charms of Ayesha could hardly have swayed the heart of Mahomet
Washington Irving admits: Perhaps he (i.e. the Prophet) sought, by this alliance, to grapple Abu Bekr still more strongly to his side.
Rev. W. Montgomery Watt states: Since Muhammad had a political aim in nearly all his marriages, he must have seen in this one a means of strengthening the ties between himself and Abdu Bakr, his chief follower.
It is admitted by all scholars that A'isha occupies a prominent place amongst the most distinguished traditionists and hundreds of traditions are recorded as having been reported by her from the mouth of the Prophet. She was often consulted on theological and juridical subjects.
That shows why the Prophet under Divine inspiration was eager to accept A' isha into his harem at that age. No other person could have proved that useful for his Mission.
So in this marriage with A'isha there was a desire to cement the bonds of friendship with Abu Bakr as well as the desire for propagating the teachings of Islam, particularly delicate matters relating to women folk.
It appears that the Holy Prophet must have noticed early the precocious nature of A'isha. So in this marriage with A'isha nothing of the sensual was involved.
You might be knowing what is usually said about the marriage of Zainab bint Jahsh - the cousin of the Prophet (Sall Allah-o-alaihe wa sallam) and widow of Zaid ibn Ha'ritha. The Qur'an first says: "Zawwajna kaha" i.e. "We (Allah) gave her in marriage (to Muhammad). Then the reason is noted: that Zainab was NOT the wife of Muhammad's son, as he had no sons. Zaid was only 'adopted". The Arab considered that a prohibited category. So the custom had to be changed by the Prophet himself, as no other one could effectively do that after him. Similar is the case of Umm al Momineen A'isha (R.A.). For a bad omen, Arab considered Shawwal a month prohibited for marriage. A'isha's betrothal and departure from her parent's house took place in Shawwal. This changed the ridiculous custom. Another bad custom was that the Arabs did not give the hands of their daughters to those who they "called" their brothers (though were not actual brothers). This was the objection raised by Abu Bakr himself, when the Prophet made the proposal. The Prophet (Sall Allah-o-alaihe wa sallam) then told him that a brother in faith was not an actual blood brother to be placed in the prohibited category.
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