The truth is that when you study the Sunni books of history and hadith, you find that there is no conclusive or clear proof or evidence that she was 6 or 9 when she got married; on the contrary, a range of possible ages is offered when you actually go beyond the myths, gossip and hearsay and actually examine the history. Here are ten reasons to reject the myth of the marriage to a 6 year old.
Several books of hadith report that Aisha was married to the Prophet at age 6 and her marriage was not consumated until age 9. Most of quotes on this have come from one man, Hashim bin Urwah, who was the last narrator of this chain. However, he did not narrate this information when he was a famous teacher in Madina, but only after he moved to Iraq in his seventies. By that stage, even his own student, Malik ibn Anas had said that his narrations should be rejected as unreliable and suspect.
According to hadith in Sahih Bukhari and Muslim, Aisha is said to have joined Muhammad on the raid that culminated in the Battle of Badr, in 624 CE. However, because no one below the age of fifteen was allowed to accompany raiding parties, Aisha should have been at least fifteen in 624 CE and thus at least thirteen when she was married following the Hijra in 622 CE.
Ibn Hishams commentary on Ibn Ishaqs Sirat Rashul Allah, the earliest surviving biography of Muhammad, records Aisha as having converted to Islam before Umar ibn al-Khattab, during the first few years of Islam around 610 CE. In order to accept Islam she must have been walking and talking, hence at least three years of age, which would make her at least fifteen in 622 CE.
Tabari reports that Abu Bakr wished to spare Aisha the discomforts of a journey to Ethiopia soon after 615 CE, and tried to bring forward her marriage to Mut`ams son. Mut`am refused because Abu Bakr had converted to Islam, but if Aisha was already of marriageable age in 615 CE, she must have been way older than nine in 622 CE.
Tabari also reports that Abu Bakrs four children were all born during the Jahiliyyah, the pre Islamic period, which could be said to have ended in 610 CE, making Aisha at least twelve in 622 CE.
According to Ibn Hajar, Fatima was five years older than Aisha. Fatima is reported to have been born when Muhammad was thirty-five years old, meaning Aisha was born when he was forty years old, and thus twelve when Muhammad married at fifty-two.
According to the generally accepted tradition, Aisha was born about eight years before Hijrah. However, according to another narrative in Bukhari (Kitaab al-Tafseer) Aisha is reported to have said that at the time Surah Al-Qamar, the 54th chapter of the Quran , was revealed, I was a young girl, a jariya. The 54th Surah of the Quran was revealed nine years before Hijrah. According to this tradition, Aisha had not only been born before the revelation of the referred Surah, but was actually a young girl, not even only an infant at that time. So if this age, of jariya, is assumed to be 7 to 14 years then her age at the time of marriage would be 14 to 21.
A famous Sunni imam, Ahmad ibn Hanbal, reports in his Musnad, that after the death of Khadijah, "Khaulah came to the Holy Prophet and advised him to marry again. She had two propositions for the Prophet: Either Muhammad could marry a virgin (bikr), or he could go for woman who had already been married (thayyib)". Khaulah named Aisha for a virgin (bikr). It is common knowledge that the term bikr in the Arabic language refers to a well formed lady and not to a 9 year old, playful, immature girl. If she were nine, the word used by Khaulah would have been jariyah and not bikr.
According to many Ahadith in Bukhari, it is believed Aisha participated in the battle of Badr and Uhud. Also in Bukhari (Kitabul-maghazi) Ibn `Umar states that the "Prophet did not permit me to participate in Uhud, as at that time, I was 14 years old." But on the day of Khandaq, when I was 15 years old, the Prophet permitted my participation. So if it was not allowed to participate in Uhud for people younger than 15, then Aisha would be at least 15 in those battles, making her age at least 13 to 14 at the time of marriage.
According to almost all the historians, Asma the elder sister of Aisha, was ten years older than Aisha. It is reported that Asma died in the 73rd year after migration of Muhammad when she was 100 years old. Now, obviously if Asma was 100 years old in the 73rd year after Migration to Medina, she should have been 27 or 28 years old at the time of migration. If Asma was 27 or 28 years old at the time of hijrah, Aisha should have been 17 or 18 years old at that time. Thus, Aisha if she got married in the first or second year after the Hijrah, as is commonly reported was between 18 to 20 years old at the time of her marriage.
So in conclusion, we are doing our Holy Prophet a great disservice. We should be defending the honour, character and reputation of our Holy Prophet and reject these claims that he married a 6 year old child, whether they appear in non-Muslim or Muslim books. The evidence is just not there
The Truth About Shias: Myths about the Holy Prophet - Part 2: Marriage to Bibi Aisha