I really do believe that life begins at 40! I embraced my 40th Birthday rather than fear it and think that the reasons for me personally are:
My children are teenagers (Good kids) and so are quite independant, which gives me more free/quality/me time.
We are more financially stable as we have been married for quite a few years.
I am more confident - this comes with age I think.
I have more life experience and and so can deal with situations in a more measured and informed way.
I am in a stable relationship.
I am more aware of my personal attributes and have actually recognised that I get more male attention now than I did when I was younger.
I love being 41 and a half and although I don't really want to have wrinkles etc, I know that I have earned them!