I have to agree that I believe women are at their best in the early thirties. At least, I am in my early thirties, so, while I can't speak for late thirties and forties, I can say that compared to my twenties, I feel and look better than I ever have.
I wonder if this also has to do with self confidence, becoming more secure with who I am as a person and my own contribution in the world. As I get older, I seem to increase and confidence/self worth, security, as well as gain a better appreciation and understanding of my place in the world. That sort of stability really makes a difference on self worth and how one takes care of oneself.