Why did Prophet Muhammad marry lady 'Aisha when she was only 9 years old?

'Aishah, may God be pleased with her, narrated that the Prophet (P) was betrothed (zawaj) to her when she was six years old and he consummated (nikah) his marriage when she was nine years old, and then she remained with him for nine years. (Saheeh al-Bukhari, Volume 7, Book 62, Number 64)
For instance, it would be silly to ask, what kind of milk did they drink 1400 years ago or what kind of engine they used back then, therefore one must first understand that things 1400 years ago were very different from now, times have changed and so have humans.
1400 years ago it was very common for women to marry at a young age, It is a historical fact that girls from the ages of 9 to 14 were married off in Europe , Asia, Africa and America.
Saint Augustine – 350 AD, He married a girl who was 10 years old.
King Richard II -1400 AD, He married a 7 year old.
Henry VIII -1500 AD, He married a 6 years old.
According to Catholic Encyclopedia "Mary the Mother of Jesus was 12 when she married 99 year old joseph".
Before 1929 Church of England ministers could marry 12 years olds in Britain.
Befor 1983 Catholic canon law permitted priests to marry off brides of at the age of 12.
A lot of people are unaware that in the United States, in the state of Delaware in 1880, the minimum age for marriage was 7 years old and in the California it was 10 years. Even now the marriage age for some states are , 14 in New York ,13 in New hampshire ,  and 12 in Massachusetts .

At the time of the Prophet Muhammad (PBUH) those who refused his message tried every method and strategy to discredit him, They were always looking for opportunities to attack him verbally or even physically. Some of this verbal abuse consisted of calling him a Magician, Crazy and Soothsayer, However, they never thought to point out object to his marriage to Aisha (RA) Why? Because this was the norm of society, meaning it was completely normal… 

Did you know that the Prophet Muhammad (PBUH), had only one wife, Khadijah, throughout the first 54 years of his life? Did you know that she was a widow?Did you know that she was 15 years older than him? He (PBUH) spent his entire youth with her, He (PBUH) committed to her until and after her death by keeping ties with her friends.

But what about Aisha (RA)? Was she a child when she agreed to marry the Prophet Muhammad (PBUH)?

First, its important to note that it is strictly prohibited in Islam to marry someone before the age of puberty (Qur'an 4:6). Note the age of puberty varies throughout different times and different places.

According to scientific references: Girls might enter full-blown puberty anytime between ages of 9 and 15. [LiveScience.com]

Also the average temperature of the country is considered the chief factor with regard to menstruation and sexual puberty [Women: An Inatorical, Gynecological, and Anthropoligical Compendium, Volume 1, Lcrd, & Bransby 1988, p563]
So was Aisha a child? No she was not! So what was the wisdom behind this marriage? God Always knows best, But the marriage of Aisha (RA) was an instruction from God, Aisha (RA) narrated to us more than 2200 hadiths (sayings) of Prophet Muhammad (PBUH).

Aisha (RA) was exceptionally intelligent, and since she married the prophet Muhammad (PBUH) at a young age, she was able to learn so much from him. She later played a important role as a teacher, expert and interpreter of Islam, so there was much wisdom behind this marriage! For those who do no know prophet Muhammad (PBUH), Prophet Muhammad (PBUH) didn't come to seek wordly pleasure, but he warned us against them! His companion described him a man of morality and they said he was more shy than a virgin in seclusion, When believers asked him an awkward questions, he would cover his face out of shyness, He taught us that each religion is known for a kind of morality and morality of Islam is Shyness/ Modesty.

, [02.12.19 19:28]
(( Say, "I am only a man like you, to whom has been revealed that your god is one God. So whoever would hope for the meeting with his Lord _let him do righteous work and not associate in the worship of his Lord anyone.")) Qur'an 18:110

In the middle ages women used to marry at 9 and 10 years old as said by the Canadian historian Margaret Wade Labarge who died in 2009
that's because saharian girls get mature early

Benny SLex
in (Internet Child Pornography :  causes, investigation, and prevention)
page 10 .

.The head of judges in the 7th district of Chicago called Richard A. Posner 
and Proffesor Katharine B. Silbaugh in Boston university departement of Law
they both said in their book (A guide to America's sex Laws) page 44 that 
"the legal allowed age of marriage before 1900 years was 10 years and was very normal at that era but now the age has changed" 
they added that "Most states in America enrolled the legal age of marriage in the 19th century as 10 years"

The Scottish law before 1900 years as Sir John Comyns and Steward Kyd state in their book (Digest of the Laws of England) volume 2. Page 73 that
"The law dictated that the legal age of marriage of girls is at the age of 9 and upon this a 9 year old girl at that age was mature enough for marriage but law never allowed marriage before 9 years old"

The American Proffesor Sociologist Anthony Joseph said that "In the midst of the 60s marriage was allowed at the age of 10" ; the American Law center prescribed the same 
(Opposing hate speech) page 10

Also Mike A. Males in his book (Teenage Sex and Pregnancy. Page 40) : said that 
"Most states in America were specifying the age of marriage from 10 to 12 except Delouir state which allowed the age of 7 according to the British Law"

Also the writer Martha Rosenthal stated in her book Human Sexuality : from cells to society) page 422
that Mike mentioned above was right and that people used to marry in Diliour state in America at the age of 7

Also the writer Merril D. Smith in her book  (Encyclopedia of Rape) page 40 she mentioned the following 
"From previous eras to present ones many societies attempted to secure children from rape and sex deterioration, though sex détérioration may differ from a era to another and from a place to another..."
till she said " but historically speaking the age of marriage was allowed from 10 to 13. And that was according to the time and culture of those societies. and each society differs in puberty"

Also the writer Paula S. Fass said in his book (The Ruteledge History of Childhood in the Western World) page 235 that " the legal age of marriage in France was 10 in the 19th century then was raised to 13.