The custom of marrying off children at a very young age compared to today

Shaykh Mufti Muhammad Sa`eed khan in his book " Kamseen bacchiyo ki shadiyan " ( the marriage of adolescent girls) " brings some effective points to counter non Muslim arguments who don't mind their small children watching porn and sleeping with 300 guys and girls before the age of 18 but mind when Muslims marry their young children realising they are physically and mentally capable to marry .  He also quotes many such narrations and incidents were similar marriages took place as that of our mother `Aisha's . One should note that Islam legalises marriage based on "ability" and not "age" for humans differ from each other based on biological, regional and other aspects that allows some to mature earlier than their counterparts.
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1. The period of that time and its surrounding is to be taken into consideration before objecting to anyone among the Muslims marrying their daughters at 6 or 9 or 12 .

2. Mufti Sa`eed khan sahab gives the example of the buildings during the prophet's time and for a good time thereafter were only max of 2 floors. Yet no one blames them for that why? 

3. The shaykh quotes ample examples of various incidents wherein the quraish and other `arab and ajam married their sons off at the age of 10 and how they would enter battle at the age of 13 or 12 or 14 years as was the norm. He gave the example of the famous sahabi `amr ibn al-`aas the conqueror of egypt who married at the age of 10 while he wasn't a muslim [Ref: Siyar `alam]

4. `Abdullah ibn `amir the son of the sahaabi `aamir ibn kareez was 12 years old when he married and he had a son the very coming year of marriage so the difference between his son abdur rahman and `abdullah ibn aamir was of 13 years only.  [Ref: Siyar `alam under `abdullah ibn aamir 7, /3 page 19; ] 

5. the uncle of the believers Mu`awiyah married his daughter Hind to `Abdullah ibn `Amir when she was 9 and this way approx 25+ years after the prophet's demise [Ref: Tareekh ad-Dimishq under the category of Hind bint Mu`awiyah ibn Abi Sufyan , No. 9781 page 138; also see siyar `alam nubala 6/3/page 18] 

6. Urwah ibn zubair (rahimahullah) the senior most taba`ee - son of the famous Zubair ibnul-`Awwam radhiallahu `anhu and Asma bint Abi Bakr . He married his cousin (paternal uncle's daughter) named Fatima bint Mundhar when she was 9 [Ref: Ibn `Adi's Al-Kamil under Muhammad ibn Ishaaq ibn Yisaar, Madani, raqam 1623;  7/256] 

And many more such incidents are there even back then and many even today. A very important thing to note is that all of these families and households were royals governors and famous people thus if they had no issues marrying their daughters and sons who matured way more quicker than today then why should those who come 14 centuries after them object to what they did in their era?

Download the whole book : https://ia801504.us.archive.org/3/items/KumsinBachonKiShadiyanByShaykhMuftiMuhammadSaeedKhan/Kumsin%20Bachon%20Ki%20Shadiyan%20By%20Shaykh%20Mufti%20Muhammad%20Saeed%20Khan.pdf