Why is Forty....Fabulous?

As I am recovering from yet another friend's truly Fabulous Forty Birthday celebration last weekend I am wondering...what is it that makes "40" such a significant year? 

Some people FEAR the age of forty as it is signal that they have (statistically speaking anyway) fewer days ahead of them than they have behind them.  40 also seems to be the time that you are truly become an "ADULT" you may be able to claim that earlier but by the time you are 40 people don't even put the young in front of the adult...you are quite simply GROWN!!  With the achievement of the 4th decade, there is also the big fear that your days of youthful indescretion lie behind you.  You have responsibilities...you need to start thinking (seriously) about 401Ks, pensions and the likely lack of social security checks for our generation.  And if you have a family you are likely dealing with impending adolescence of your kids and (UGH) figuring out how you will pay for their college education.  There are mortgages (maybe at this point of a multiple nature), a mountain of bills and worry on every hand. 

To me this is not a bad thing.  I have always felt like an "old soul" trapped in a younger body so I actually look forward to my chronologic age catching up with my spiritual and emotional age. Forty to me is fabulous because it is when you reach that level of comfort with yourself and your relationship to the world around you.  By 40 you hopefully have:
-   figured out who your REAL friends are.
-  found a spiritual center
-   made peace with your parents and your childhood (however happy or unhappy they were)
-  made a BUNCH of mistakes
-  recovered from said mistakes and realized that is how you learn
-  Been in Love!
-  had your heart broken
-  experienced great loss
-  experienced an amazing accomplishment
-  laughed until you peed
-  been somewhere amazing that took your breath away

I have had many of the experiences above...and in this last year as I march up towards 40 I want to figure out what experiences I want and how I want my life to be going forward.  Unlike a lot of my friends I am truly looking forward to being fabulous and 40...I feel it will be the birth of the real me!