By being the Wali of their children, parents have a duty to protect them until they reach the age of majority. After then, their permission only becomes important if there is an issue about safety. If the father is really worried about something for good reason, it would be wrong (makruh) to ignore his concerns. But if he does not have a valid reason or he is just being excessively conservative, a grown up boy or girl is not bound to his/her father’s permission. That means he does not have a right to prohibit his children and they do not have to obey him.
Answering to fathers’ questions is certainly a good deed and Muslims are commanded to be good to their parents. Allah the Almighty commands:
“Your Lord decrees not to worship any but Him and to be good to your parents; if one or both of them reach the old age with you, do not even say to them ‘Uff!’ and do not stay indifferent to them. Speak honoring words to them.” (al- Isra 17:23)