There might be nothing wrong with it at the moment — you probably have some terrific investments. But a retirement account has to shift its balance as you age.

When we’re young, it’s best to be aggressive. Stocks, stocks and more stocks will ensure that your portfolio grows. Even though stocks rise and fall, over time they are the best way to build a nest egg. But as you get closer to retirement, you need to shift your portfolio to reduce the amount devoted to those volatile stocks, since a plunge at a moment when you can least afford it could slash the savings that you need to live on.

So at that stage in your life, experts suggest you load up on bonds, which tend to fluctuate less, and extremely stable investments like money market funds. Bonds don’t earn as much as stocks do, generally, and they can certainly decline as investments when markets slump. But they won’t plunge nearly as much as stocks — and sometimes they even go up when stocks go down.

“If you are still working, then you can be very aggressive,” said Tom Fredrickson, a certified financial planner in Brooklyn. When stocks go down, being invested in stock funds means you are buying them more cheaply. That kind of risk-taking should slow considerably within five years of ending your employment, he warned. “At the beginning of retirement, you shouldn’t be 100 percent in stocks.”

He explained the math: If you suffer a stock market fall that reduces the value of your stock holdings by 50 percent, “you’ve got to make 100 percent to get back your 50 percent.” And if you are pulling money out of the account to pay for day-to-day expenses by then, “you’re going to go broke quickly.”

There is an old rule of thumb for asset allocation: Subtract your age from 100, and the resulting number is the percentage of your investments you should have in stocks. Because people live longer than they used to, and because investments like Treasury bonds don’t pay as much as they used to, rule-of-thumb watchers have revised the number to subtract from 110 or even 120.

Depending on your stomach for risk, you might hold on to your stocks even longer.

Staying in the stock market very late in the game is smart only if you’re wealthy enough, or, say, have the stability of a pension or the expectation of an inheritance, and “the market is just a game for you,” Mr. Fredrickson said.