Age, Hair, and Gender

As a graying, balding male, it was interesting to see yesterday that a friend of mine, a female professor here, decided to stop coloring her hair. While we as a culture worship youth as a general policy, the choice to allow one's grays to show is a much bigger deal for women. Why is this? It is especially interesting when you are in a job where authority is an issue and age generally brings with it, respect that is wrongly denied female instructors. What is at issue with allowing oneself to be a woman in this culture with gray hair that is not an issue for men?