For me at 40 there has been some realization that some of the things that I wanted in life just aren't going to happen.
I've been really fortunate where I work, I have always worked with "old guys" that were 50+. They ALL bitched about wanting to retire ASAP and "fuck this place." Several took me aside and said, "You don't want to be stuck here til age 65." I tried to look ahead and save for retirement.
I suffered a stroke at 38 that put me out of work for 5 months. I really questioned if I could get back to where I was. Without 20/25 corrected vision my career is over. I made it back to work but 40 has really highlighted that I may need to bail out of my career at 50-55. Just focus on getting a job I enjoy rather that attaining the highest income possible.