The oldest of the millennials are now only five years from the big 4-0 and (time being inexorable) the remainder are following close behind. Meanwhile, the GenXers are slipping into their middle ages (and I don't mean the Society for Creative Anachronisms).

When you hit 40, you are way past the day when you can get hammered on a weeknight and get to work in the morning, and those 80-hour weeks that you managed so blithely whilst a whippersnapper now burn you out faster than an iPhone 7 battery.

What's more, if you're 40, you've probably got a personal life; you may have some kids; you may be divorced and dating (the horror, the horror...). More important, you're too old to have an identity crisis and too young to have a mid-life one.

You are, however, smack dab in the right time to have a career crisis, because by now you've 1) paid your dues, 2) gained momentum, and 3) can see clearly where you're likely to end up ten or twenty years down the line, if you stay the course.

So, this is the time when you need to ask this all-important question: Is this job and this career where I can do my best work?

Even if you've got responsibilities, there's no better time to start a business than when you're in your 40s. You know the score, you know the game, you know how people think, and chances are you've stockpiled some cool ideas for your own business.

Similarly, if you've been an entrepreneur all your life and you haven't yet it big, hitting 40 is the perfect time to decide whether you might actually be happier working for somebody else. And unlike those who hold out till their 50s, you're still young enough to get hired.

This is something that I learned by accident. When I hit 40, I had a cushy corporate job, with an expense account, plenty of world travel and interesting tasks, like big budget branding and negotiating billion-dollar acquisitions.

Nevertheless, I wasn't happy with the job and couldn't stand the bureaucracy and the woodenheaded management, etc., etc. And to make matters much worse, I was a complainer. Yes, as embarrassing as it is to admit today, I was a complainer. Ugh.

Anyway, I was whining about work hassles one day to a super-smart guy--a marketing contractor with his own busines--and he says: "Geoff, if you don't think you can do your best work here, you should leave. Life's too short to waste doing stuff you're not proud of."

His words were like a slap of cold water. I realized that I was complaining all the time because I knew--KNEW--in my heart of hearts that I could be something more than a mid-level exec in a huge company

At the time, I didn't know where I was headed and it took me a year to put aside enough money and vacation time to float a career change... But I knew that where I was headed wasn't where I wanted to go.

And here's the thing... within two years after I left, I was making far more money, spending far less time at work and enjoying myself more each day. Yeah, I miss the expense account, the four-star hotels, and the paid vacations. But I'm glad I made the leap.

There's nothing better in your life and career than knowing that you're truly doing what you were meant to do.  And turning 40 is a great time to decide to live a genuine life.