My mother told me that when she turned 40 it was a liberation from a life of caring about what other people think, or what other people expect. It was a time to free yourself from the burden of others' expectations and live your life for yourself. And why couldn't she have done this before? Who knows? Maybe it is simply a second coming of age; a coming to consciousness when you suddenly realise that your own self is different from a media personification of woman or the woman that the people around you would like you to be.
For me, I woke that morning feeling somewhat alone, even invisible. But, ah! What freedom!