Hi
If you cross reference Quranic ayaat then age of Nikaah is that of maturity (gain of strength ) and that is 40 yrs
004.006
YUSUFALI Make trial of orphans until they reach the age 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 Allah in taking account.
PICKTHAL Prove orphans till they reach the marriageable age; then, if ye find them of sound judgment, deliver over unto them their fortune; and devour it not by squandering and in haste lest they should grow up Whoso (of the guardians) is rich, let him abstain generously (from taking of the property of orphans); and whoso is poor let him take thereof in reason (for his guardianship). And when ye deliver up their fortune unto orphans, have (the transaction) witnessed in their presence. Allah sufficeth as a Reckoner.
SHAKIR And test the orphans until they attain puberty; then if you find in them maturity of intellect, make over to them their property, and do not consume it extravagantly and hastily, lest they attain to full age; and whoever is rich, let him abstain altogether, and whoever is poor, let him eat reasonably; then when you make over to them their property, call witnesses in their presence; and Allah is enough as a Reckoner.
017.034
YUSUFALI Come not nigh to the orphan's property except to improve it, until he attains the age of full strength; and fulfil (every) engagement, for (every) engagement will be enquired into (on the Day of Reckoning).
PICKTHAL Come not near the wealth of the orphan save with that which is better till he come to strength; and keep the covenant. Lo! of the covenant it will be asked.
SHAKIR And draw not near to the property of the orphan except in a goodly way till he attains his maturity and fulfill the promise; surely (every) promise shall be questioned about.
046.015
YUSUFALI We have enjoined on man kindness to his parents In pain did his mother bear him, and in pain did she give him birth. The carrying of the (child) to his weaning is (a period of) thirty months. At length, when he reaches the age of full strength and attains forty years, he says, "O my Lord! Grant me that I may be grateful for Thy favour which Thou has bestowed upon me, and upon both my parents, and that I may work righteousness such as Thou mayest approve; and be gracious to me in my issue. Truly have I turned to Thee and truly do I bow (to Thee) in Islam."
PICKTHAL And We have commended unto man kindness toward parents. His mother beareth him with reluctance, and bringeth him forth with reluctance, and the bearing of him and the weaning of him is thirty months, till, when he attaineth full strength and reacheth forty years, he saith My Lord! Arouse me that I may give thanks for the favour wherewith Thou hast favoured me and my parents, and that I may do right acceptable unto Thee. And be gracious unto me in the matter of my seed. Lo! I have turned unto Thee repentant, and lo! I am of those who surrender (unto Thee).
SHAKIR And We have enjoined on man doing of good to his parents; with trouble did his mother bear him and with trouble did she bring him forth; and the bearing of him and the weaning of him was thirty months; until when he attains his maturity and reaches forty years, he says My Lord! grant me that I may give thanks for Thy favor which Thou hast bestowed on me and on my parents, and that I may do good which pleases Thee and do good to me in respect of my offspring; surely I turn to Thee, and surely I am of those who submit.