Assalaamu alaykum. I have come to know from Surat An-Noor that children, when they become adult, should seek permission to enter their parents' rooms. But in my country and culture, we do not seek permission at all. That is kind of an acceptable thing. Moreover, in most cases, the door of the parents' rooms are kept open, and seeking permission becomes unnecessary. I do not know what I should do in these cases.

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.

A Muslim should adhere to the commandments of Allah that were stated in the two Revelations (the Quran and the Sunnah) and renounce the customs that violate them. This includes adhering to the two verses in which Allah says (what means): {O you who have believed, let those whom your right hands possess and those who have not [yet] reached puberty among you ask permission of you [before entering] at three times: before the dawn prayer and when you put aside your clothing [for rest] at noon and after the night prayer. [These are] three times of privacy for you. There is no blame upon you nor upon them beyond these [periods], for they continually circulate among you - some of you, among others. Thus does Allah make clear to you the verses; and Allah is Knowing and Wise. And when the children among you reach puberty, let them ask permission [at all times] as those before them have done.} [Quran 24:58-59]

The child who has not yet reached puberty should seek permission at the three times mentioned in verse 58. As for those who are grown up and have reached the age of puberty, they must seek permission at all times, as Allah says (what means) {And when the children among you reach puberty, let them ask permission [at all times] as those before them have done.} [Quran 24:59]

'Ataa’ ibn Yasaar, may Allah be pleased with him, narrated that a man asked the Prophet “O Messenger of Allah, shall I ask permission before I enter upon my mother?” He said, “Yes.” The man said, “I live with her in the house”. The Prophet said, “Ask her permission.” The man said, “I am her servant.” The Prophet said, “Ask her permission. Do you want to see her naked?” He said, “No.” He said, “Then ask her permission.” [Maalik]

If it is known that the parents give permission to their children to enter, then they may enter when there is a sign, such as when the door is left open and the like, but if they closed the door or they forbid the children to enter upon them without permission, then the children must not enter until they are given permission.

The evidence about this is that Ibn Mas‘ood, may Allah be pleased with him, narrated, “The Prophet said to me, ‘The sign that you have been permitted to come in is that the curtain is raised or that you hear me speaking quietly, unless I forbid you.’” [Muslim]

Sharh An-Nawawi on Saheeh Muslim (14/150) reads:

“It is proof that it is permissible to rely on a sign with regards to permission to enter. If the ruler or the judge or the likes of them make raising the curtain that is on their door a sign of permission to enter for all people or for a particular group or for a specific person, or if they choose another sign, then it is permissible to rely on it and enter without permission. The same thing applies if a man puts that sign between him and his servants ... and his grown up children and his family, and when he lets the curtain down, then there is no entry upon him except with permission, and when he lifts the curtain, it is permissible to enter upon him without permission.”

Allah knows best.