Marriage age in Islam
In Islam it’s advised to test the judgement of the person in order to decide if they are able to face marriage or not and it’s also advisable to get married early. 

Only recently the marriage age was brought to 18 years old. However even in Europe not long ago, girls used to get married as early as 14 years old. 

So from my opinion we can’t judge the customs or marriage age of more than 1400 years ago or seek to apply terms that were introduced recently.

4:6 Nasa

‎وَابْتَلُوا الْيَتَامَىٰ حَتَّىٰ إِذَا بَلَغُوا النِّكَاحَ فَإِنْ آنَسْتُمْ مِنْهُمْ رُشْدًا فَادْفَعُوا إِلَيْهِمْ أَمْوَالَهُمْ ۖ وَلَا تَأْكُلُوهَا إِسْرَافًا وَبِدَارًا أَنْ يَكْبَرُوا ۚ وَمَنْ كَانَ غَنِيًّا فَلْيَسْتَعْفِفْ ۖ وَمَنْ كَانَ فَقِيرًا فَلْيَأْكُلْ بِالْمَعْرُوفِ ۚ فَإِذَا دَفَعْتُمْ إِلَيْهِمْ أَمْوَالَهُمْ فَأَشْهِدُوا عَلَيْهِمْ ۚ وَكَفَىٰ بِاللَّهِ حَسِيبًا

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 Allah as Accountant.