The Quran

Preface

وَابتَلُوا اليَتامىٰ حَتّىٰ إِذا بَلَغُوا النِّكاحَ فَإِن آنَستُم مِنهُم رُشدًا فَادفَعوا إِلَيهِم أَموالَهُم ۖ وَلا تَأكُلوها إِسرافًا وَبِدارًا أَن يَكبَروا ۚ وَمَن كانَ غَنِيًّا فَليَستَعفِف ۖ وَمَن كانَ فَقيرًا فَليَأكُل بِالمَعروفِ ۚ فَإِذا دَفَعتُم إِلَيهِم أَموالَهُم فَأَشهِدوا عَلَيهِم ۚ وَكَفىٰ بِاللَّهِ حَسيبًا

Yusuf Ali

Make trial of orphans until they reach the age¹ of marriage; if then ye find sound judgment in them, release their property to them; but consume it not wastefully, nor in haste against their growing up. If the guardian is well-off, Let him claim no remuneration, but if he is poor, let him have for himself what is just and reasonable. When ye release their property to them, take witnesses in their presence: But all-sufficient is God in taking account.²

The age of marriage is the age when they reach their majority.
It is good to take human witnesses when you faithfully discharge your trust; but remember that, however fully you satisfy your fellow-men when you give your account to them, their is a stricter account due from you to God. If you are righteous in God’s eyes, you must follow these stricter standards.