If you're among the roughly 20% of Americans who have access to a 401(k) or 403(b) plan at work but don't bother to participate, I have a question for you:

What are you thinking?!

Actually, I don't really need to ask. I spent years working for a retirement plan provider and I've seen tons of survey data. Most people's reasons can be boiled down to one of five or six excuses. Two of the most common have to do with age:

  • I'm too old. Saving won't make much of a difference.
  • I'm young. I'll deal with it later.

Let's look at each in turn.

Never too old to get started
Many folks nearing retirement age feel that it's too late to save enough to make a difference. But "enough to make a difference" doesn't have to be millions of dollars -- it can be as little as $100,000. That might still seem like a lot, but if you've got five or six years of work left and you're willing to put some effort into this, you should be able to get there without too much trouble.

Why bother? Well, consider that lots of retirees take part-time low-wage jobs to help supplement Social Security. Working 15 hours a week at the local coffee shop for minimum wage will pay you about $4,500 a year after taxes. If you hold that $100,000 in cash and take the recommended 4% a year, you'll get close to that -- without having to come home smelling like cappuccino several times a week. And if you were to invest your nest egg in solid income-producing blue-chip stocks like Johnson & Johnson (NYSE:JNJ) and United Parcel Service (NYSE:UPS) instead of holding it in cash, you could safely take considerably more -- and get some capital appreciation, too.

Start young, live large later
On the other hand, if you're young and think the above lets you off the hook, think again. You've probably heard that compounding -- reinvesting your investment gains and watching the whole pile grow faster over time -- is one of the keys to amassing a fortune. And the power of compounding is even more dramatic in a tax-advantaged retirement savings plan, because you don't have to send part of the pile to the government every year. Of course, the earlier you start investing, the more compounding you get. Consider this: if you were to start putting $500 a month -- well below the limits -- into a stock fund in your workplace plan or IRA at age 25, continue for 10 years, and then stop, doing no further investing whatsoever -- you'd have $2,163,834 at age 65, assuming the fund you chose matched the market average return of 10.5% after expenses.

Now think about how big that pile would be if you collected your employer's match, maxed out your annual contributions, and ramped up your contributions as the legal limits (and your income) went up over time. Let's assume that between your workplace plan, your IRA, and your employer's match (also known as free money), you average $20,000 a year in tax-deferred savings over the next 40 years. (That's probably low, but it's just an example.) Stick that in an index fund that returns 10.5% a year on average, and you'll have more than $11 million when you're 65 -- if you start at age 25. Think 4% of that a year might beat Social Security -- assuming Social Security is even around then?

The upshot
It's pretty simple -- save for retirement! Workplace savings plans make it easy -- the money is taken out pretax so you hardly miss it, the options are usually pretty simple to understand (and if not, hit the Fool's friendly discussion boards and ask for help), and the impact on your retired lifestyle can be huge -- no matter your age.

For more helpful advice on navigating your way to a successful retirement -- and on maximizing your financial freedom once you get there -- check out The Motley Fool's Rule Your Retirement newsletter. Each month's issue is packed with practical, easy-to-understand guidance and ideas from some of the best minds in the business. Check it out free for 30 days -- there's no obligation to buy.

Fool contributor John Rosevear has no position in the stocks named in this article. Johnson & Johnson and UPS are Income Investor recommendations. The Motley Fool has a disclosure policy.