Turning 65 is a magic number for many Americans. That's the age at which people expect to retire and stop working. For some, retirement can come even sooner, maybe at 60 or even 55.

There are even plenty entrepreneurs who are fortunate enough to build up and then sell a business when they're just in their 40s. That's a dream come true for many people, who envision retiring to a life of leisure sitting on a beach or playing endless rounds of golf.

But I have some bad news for you. If you are someone who works with your brain, you can never really retire and you shouldn't. Let me explain.

I have worked with several CEOs who have had great success in their careers where they got the chance to sell their businesses for enough money where they would never have to work again. But there's a catch: you can only play so many holes of golf before you realize you aren't going to qualify for the PGA or even beat your 10 handicap. Plus, most of your buddies are still working, so you don't have anyone to play with. In short, this kind of retirement gets boring really quick.

And that's a problem because there's plenty of scientific evidence that if you want to live a long and fulfilling life, you need to keep yourself engaged and mentally stimulated--something that goes beyond Sudoku and crossword puzzles.  There was a study done of a workforce of machinists and the average amount of time they were paid a pension after retirement.  The average payment period was 18 months - in other words, they went home, got bored and decided to die.

It's actually a very sad day when true "retirement" comes, which I think of when you can't physically or mentally do the things you love any more.

That's why I want to challenge the notion that people should think about retirement as some kind of end goal. Rather, I'd like you to think of it more as a shift in your thinking and how you spend your time.

The options are endless. For some entrepreneurs, a retirement shift could be jumping back into the mix and starting up a different company, or even more than one. For others, it might mean lending your advice and wisdom to other entrepreneurs and perhaps even investing in their companies.

There's also the potential to serve on advisory boards for both for-profit and non-profit organizations. Finding ways to invest your time in different volunteer efforts can be extremely rewarding and engaging even if you aren't being paid anything for your efforts.

Again, the point is that we should all think very differently when it comes to how we want to think about how we spend our time when we've achieved a certain level of success.

Now this doesn't mean that you should have time for some fun, after all, you've earned it.  So generally, you'll work a lower proportion of your time and increase the amount of recreation.  That's part of the shift.

I think this can also have a profound affect on how we as professionals think about our time even as we work in our careers. Rather than planning for a retirement end goal, I think it's healthier to think more about taking a series of sabbaticals in your life. What if you took six months off every few years to travel, learn to scuba dive, or anything you are passionate about that gets your juices flowing versus waiting until someone hands you a gold watch to kick off your official retirement? I think you'd be happier and healthier as a result.

So, when you think about your own retirement, think of it more as a shift than an ending. And then think about what kinds of changes you can make today to help you map out the kind of life you want to lead for the rest of your career rather than waiting until the end to figure it all out.