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It depends. The right answer in your case will depend on a number of key factors. These include, among others, your income and assets, your attitude toward risk, whether you have access to an employer-sponsored plan at work, the age at which you plan to retire, and your projected expenses during retirement. But it's possible to lay down some guidelines that may be of help to you.
The conventional wisdom has traditionally been that you should invest aggressively when you're young and then move gradually toward a more conservative approach. By the time you retired, you would probably end up with a portfolio made up mostly of high-grade bonds and other low-risk investments. However, the retirement landscape has changed dramatically in the past 20 years or so. As a result, many of our basic assumptions about retirement planning have been overturned.
The dwindling number of traditional pension plans and concerns about Social Security have led people to take greater responsibility for their own retirement. Investing more aggressively over the long term has become common as people realize that, without anyone else to take care of them, they need to build the largest retirement nest egg they possibly can. In fact, many people these days primarily use growth vehicles (e.g., certain stocks and mutual funds) for their investment portfolios and tax-deferred retirement plans (e.g., 401(k)s and IRAs), though the proportion of stocks may still be reduced as they near retirement.
Other factors have changed the way we think about and invest for retirement as well. People tend to retire younger, live longer, and do more during retirement than they used to. With the likelihood that you will have well over 20 years of activity to fund, it may be a good idea to invest more aggressively for retirement than previous generations did. And there's no reason to switch over to fixed-income securities completely upon reaching retirement, especially with interest rates at historic lows. Because bond prices typically fall when interest rates go up, a period of rising interest rates can affect the value of your bond holdings. Many financial planners suggest that you keep a suitably balanced portfolio, including some of your assets in growth-oriented investments, even after you retire.
Don't forget to carefully consider a mutual fund's investment objectives, risks, fees, and expenses, which can be found in the prospectus available from the fund. Read it carefully before investing. All investing involves risk, including the possible loss of principal, and there can be no assurance that any investment strategy will be successful.