If we are not going to ask for the participants' names and do not want to mention the exact location of the participants in the paper, do we still need to ask for parents' permission for the under age?
Yes, in my country, it is not possible to enroll any person in a research without applying a post informed acknowledgment term. If the subject has less than 18 years old, this term has to be signed by the parents.
A lot depends on the institution you work for and probably your country's policies on this. But I am pretty certain that you will most likely have to do it based on your target population (under 18). Contact your institution's IRB office and see what they tell you.
Atefeh, if you do the research in the U.S. you will definitely need to get permission and you will have to submit your proposal to a review board to insure it has the elements of "informed consent." I imagine that you are not planning research in the United States, but the practice of providing informed consent (they get to choose and they know how the research will be conducted and used) provide you with a solid ethical (and perhaps legal) basis for your research. I believe you will also find that most places do not believe that a person under the age of 18 can provide fully informed consent so parental or guardian permission is needed.
My recommendation is to always insure informed consent, even if it is not legally required.
It is always appropriate to get IRB review and approval of data collection procedures. In some instances the nature of the questionnaire will make an important difference. When asking about sexual experience versus asking about frequency of using the internet, in circumstances where the response cannot be connected to any descriptors that would allow identification of the person responding, the IRB ruling might well be different. In one case even an anonymous question could be disturbing to a young person, but in others that risk would be considered extremely low. I have done research in schools using simple permission forms for students to have caregivers sign, and in one case actually presenting to parents what would be presented to the students, with an opt out choice for their children. The latter process allowed for use of material with students that might otherwise have been controversial. The students themselves could also opt out even if parents gave consent.