Age of marriage

وَابۡتَلُوا الۡيَتٰمٰى حَتّٰىۤ اِذَا بَلَغُوا النِّكَاحَ​ ۚ فَاِنۡ اٰنَسۡتُمۡ مِّنۡهُمۡ رُشۡدًا فَادۡفَعُوۡۤا اِلَيۡهِمۡ اَمۡوَالَهُمۡ​ۚ وَلَا تَاۡكُلُوۡهَاۤ اِسۡرَافًا وَّبِدَارًا اَنۡ يَّكۡبَرُوۡا​ ؕ وَمَنۡ كَانَ غَنِيًّا فَلۡيَسۡتَعۡفِفۡ​ ۚ وَمَنۡ كَانَ فَقِيۡرًا فَلۡيَاۡكُلۡ بِالۡمَعۡرُوۡفِ​ ؕ فَاِذَا دَفَعۡتُمۡ اِلَيۡهِمۡ اَمۡوَالَهُمۡ فَاَشۡهِدُوۡا عَلَيۡهِمۡ​ ؕ وَكَفٰى بِاللّٰهِ حَسِيۡبًا‏ ٦

And test the orphans [in their abilities] until they reach marriageable age. Then if you perceive in them sound judgement, release their property to them. And do not consume it excessively and quickly, [anticipating] that they will grow up. And whoever, [when acting as guardian], is self-sufficient should refrain [from taking a fee]; and whoever is poor - let him take according to what is acceptable. Then when you release their property to them, bring witnesses upon them. And sufficient is Allāh as Accountant.

[Al-Quran 4:6, Translation: Saheeh International]

A common misconception is that Islam allows marrying of children. It doesn't. Surah an-Nisa (The Women) explicitly states that when orphans reach 'nikah' age, test them to see if they have legal sense (رُشۡدًا), and if they do, only then is their guardian to hand over their inherited property to them. Moreover, nikah itself is a ratified covenant between two adults, as stated in ayaat 20 and 21 of the same surah: 

وَاِنۡ اَرَدتُّمُ اسۡتِبۡدَالَ زَوۡجٍ مَّكَانَ زَوۡجٍ ۙ وَّاٰتَيۡتُمۡ اِحۡدٰٮهُنَّ قِنۡطَارًا فَلَا تَاۡخُذُوۡا مِنۡهُ شَيۡـــًٔا​ ؕ اَ تَاۡخُذُوۡنَهٗ بُهۡتَانًا وَّاِثۡمًا مُّبِيۡنًا‏ ٢٠ 

وَ كَيۡفَ تَاۡخُذُوۡنَهٗ وَقَدۡ اَفۡضٰى بَعۡضُكُمۡ اِلٰى بَعۡضٍ وَّاَخَذۡنَ مِنۡكُمۡ مِّيۡثَاقًا غَلِيۡظًا‏ ٢١

But if you want to replace one wife with another and you have given one of them a great amount [in gifts], do not take [back] from it anything. Would you take it in injustice and manifest sin?

And how could you take it while you have gone in unto each other and they have taken from you a solemn covenant?

[Al-Quran 4:20-21, Translation: Saheeh International]

The Age of Aisha رضي الله عنها, the wife of Prophet Muhammad ﷺ is commonly quoted as proof of underage marriage. According to a study on Dr Khalid Zaheer's website, the age of Aisha رضي الله عنها at the time of her nikah was 18, and at the time of rukhsati she was 19 years old. 

Related Links: 

The Covenant & Its Ratification for why مِّيۡثَاقًا means a ratified covenant 

Understanding Q2:256 (No compulsion in Deen) for why رُشۡدًا implies legal sense  

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