Well it depends upon the type of hair you guys have. If have oily hair you should better wash your hair more than once a week. Probably every second day.
If you have curly hair you can skip your bath every once in a while but if you have straight hair and oil in your hair tends to get noticeable after a couple of days then it is better to wash your hair more than once a week.
Another factor that plays an important role in determining the number of times you should wash your hair is your routine. If you are working in a dirty place and perspiration is unavoidable, then you should better wash your hair daily.
If you are an African American your hair probably would be dry, so for you it would be better to wash your hair just once a week. If you wash them daily it might become more dry.
Moreover, when we are young our hair can get away with less frequent washing but when we grow old our hair can become oily (especially in teen life) and requires more frequent washing.