I agree with sofs about the maturity at 40 and the being more established and directional but I disagree that life begins at 40.
I think it must begin when you finally retire but if they keep putting the retirement age up that will never be.
I must say that as I approached 40 I looked forward to life beginning - it didn't! It stayed the same as ever, all work, some play when I had the time, same old struggles, same old pleasures...and as I approach 50 - it is no different. Perhaps if I won the lottery???
On a similar point I believe that 'life is what you make it', what you put into it you will eventually get out of it. Enjoy work, enjoy rest, enjoy play - enjoy life and eventually enjoy retirement!