salamu alaykum
Note that women back then were much more grown up then women nowdays. Women before used to be raised at young age to be a woman and be able to take care of things on her own. Resulullah svs didn't do things on his own. It was a command from Allah sva to marry Aisha r.a. infact, before the marriage Muhammad

had a dream twice of marrying Aisha ra. We all know that the dreams of the prophets is Ru'yaa, revelation. Ibrahim a.s had a dream of slaughtering his son Ismail a.s and so he answered the call, because he knew that the dreams of the prophets are a revelation and truth.
Hadith narrated by aisha r.a that the messenger

said to here: "Before I married you, Isaw you in my dream twice. An angel caried you, wrapped in a piece of silk, and told me, "this is your wife." I uncovered the face and found that it was you. i said to myself, "if this is from Allah, He will surely make it happen. (al-Bukhari, Muslim, and others).
Another hadith recorded by al_marwazi, ahmad, and others and verifyed authentic by shaikh Al-albani that Aishah r.a reported that the prophet

told her when he was near death: "indeed, what makes deeaath lighter for me is that I was shown (in a dream) that you are my wife in Jannah".
From these two hadiths we realize that both were dreams of the prophet

and therefore a command from Allah sva that it will happen. Muhammad

told one of his sahaba to write down everything he

said. The sahaba asked "even, when you are angry" and the prophet

told him "yes" that nothing comes out of his mouth but the truth. Meaning that everything that he says or does is revelation and guidence from ALlah sva.
Second, as i mentioned in the beginning that we can not compare the characteristics of a woman's condition 1400 years ago with women today. Infact, the sign of a woman's adulthood, is her menstrual cycle. Allah sva gave her a sign that she is able to bare children. Why would Allah sva give a woman the capability of baring children at age 16, 14, 12, even some at 8,9 today if she was not old enough to get married?
Even 40-50 yrs ago, woman used to get married at younger ages. Both of my grand mothers got married at age 12-13, and it was normal and common. No one complained about it until now. Today, if you are 22 and married, people look at you different like "why"?
If you are 16 and a female, no you can't because it is illegal and you are young. Although in some states in a America it is legal if the guardians sign marriage certificate. However, she is ok to sleep around at any age that pleases her/him, but not get married. Is this justice to the people?
If women were married at young ages 40-50 yrs ago, which is recent, than how can we compare the status of the woman 1400 yrs ago, subhanALlah and say that it was a little kid. We all know that it is not permissible to engage in sexual activity with someone before they reach their period, and ofcourse being your wife.
In islam, a woman can be married by her guardian without her permission but before her puberty. However, when she gets the period, she can choose if she wants to stay with the husban or devorce him. Now, Aisha r.a was married and she could have chosen not to stay with resulllah,

but she did.
Brother osmanelmais brought a good proof that Aisha r.a was engaged to someone else before islaam, but then her father Abu Bakr assideek canceled that and married her with Muhammad

.
Note also, that her parents were the ones who married her as well. If it was not a custom back then to marry at that age, her parents would not allow it. Also, you guys realize that we are talking here about marriage. The kuffar and a lot of muslims who don't know islaam or are arrogant do not realize that Muhammad

didn't marry her because of desires only. He didn't date her, and then kick her out. He married her first of all because it was a command of ALlah, second because of a benefit. She learned so many hadith, infact she is the second after abu hurayrah r.a who transmitted over few thousand hadith.
So, to go into this stuff is really silly. We have to defend our prophet

and know that he was sent as mercy to mankind.
If the kuffar ask you about this stuff and how come he

married a young girl like that, you ask them this. Ask them "Do you believe in Allah sva as the only one to be worshippped without any partner, and that Muhammad s.vs is His final messenger?" If they say,"no" then that is the end of discussion. If they do not believe that, how will they belive that it was a command from ALlah sva?
IF they bring this issue, just changed it and ask them "let me ask you what you know about islaam?"
Allah knows best
wasalam