If a man owned slave female, acquired lawfully, what would be the minimum age she should attain to make sexual intercourse with her permissible (i.e. sexual intercourse that does not incur any punishment or repentance)? Please provide Islamic scriptural evidence since everything that is not haram/impermissible is halal/permissible.
Slavery has been abolished. The Qur’an refers to the gradual freedom of slaves. Thus, the question has no relevance to today’s world. Besides, Islam did not establish the institution of slavery or its regulations. What Islam did with 7th century Arabian slavery is to wrest it out of the fibers of the society. This, it had to do with a very reluctant people. Your hypothetical question, thus, is not related to religion. The society, when they instituted slavery, had their own rules as to how treat people, slaves included. Islam put forth a way to eliminate it by raising the humanity of a slave in the minds of people, giving him rights and reminding the community of the spiritual elevation of freeing slaves.
The following verses refer to the gradual manumission of slaves and their proper treatment:
Serve Allah, and join not any partners with Him; and do good- to parents, kinsfolk, orphans, those in need, neighbours who are near, neighbours who are strangers, the companion by your side, the wayfarer (ye meet), and what your right hands possess: For Allah loveth not the arrogant, the vainglorious – Al Nisaa’ 4:36
Alms are for the poor and the needy, and those employed to administer the (funds); for those whose hearts have been (recently) reconciled (to Truth); for those in bondage and in debt; in the cause of Allah; and for the wayfarer: (thus is it) ordained by Allah, and Allah is full of knowledge and wisdom. – Al Tawbah 9:60
And what will explain to thee the path that is steep?- (It is:) freeing the bondman; -Al Balad 90:13
I hope this helps.