My mother and I were just discussing this subject last night.
I was looking through her high school year book and noticing how about 90% of the females had head fulls of hair (this included the faculty as well). The only person I can compare them to is Oprah when she used to wear her hair straight.
You do not see this type of fullness (seemingly healthy) hair on high schoolers and young adults these days. Why? Most of them relax now. Back in the late 60's, most people got their hair pressed.
Also, I can see a difference in my own hair. I got relaxers from the ages of 13-26. I stopped because no matter how well I took care of my hair, what hair "professional" I went to , what products I used- it just wasn't as strong and healthy as it used to be. (Although, I believe beauticians had a lot to do with it now that I look back on it.)So, I stopped relaxing.
Almost 7 years later, my hair is much fuller and stronger and grows at a decent pace. However, I look back at pictures of when I was a child and my natural hair is still much thicker than it is now. So, within that 13 years of relaxing, I do believe in my heart that some hair follicles were permanently damaged.
This is just my personal observation of my hair. As others have mentioned; heredity, personal practices, illness/medications also are big factors.