The dilemma: You are saving for a long-term goal such as retirement or your children’s college education, and you’re not sure how to juggle the mix of stocks and bonds to get the best return at the lowest risk.

The solution: Invest in a target-date mutual fund that does it for you, gradually changing the mix of holdings from a focus on growth in the early years to one favoring safety as you get closer to needing your money.

Sounds perfect, doesn’t it?

It’s worth considering, and there’s a raft of such funds going by various names, such as life-cycle funds, life-strategy funds, target-retirement funds, age-based funds and so on. Many are popping up in 401(k)s and similar accounts, and they’re also popular in 529 college-savings plans.

But be careful: Much as I like simple investing strategies that don’t take a lot of fiddling, there is such a thing as too much autopilot. A fund designed for people retiring in 2015, for example, may be perfect for one investor but not so good for another.

Before investing in a fund with a target date, do some independent research – or get some professional advice – to be sure the fund’s mix of assets really does suit your needs.

Interest in these funds has taken off in recent years. At the end of 2000, there were 55 such funds with total assets of about $24 billion, according to Morningstar Inc., the fund-data company. On Dec. 31, there were 173 funds with $114 billion in assets. Morningstar plans to start listing these funds in their own time categories next month.

Some of these funds hold stocks and bonds directly, while others hold shares of other funds. Generally, they all incorporate a target date selected by the investor, such as the year the holder will retire.

The idea is to emphasize stocks in the early years, in hopes of getting strong returns while there’s plenty of time to wait out market downturns. Over time, larger portions of the holdings are shifted from stocks to bonds to emphasize safety.

Vanguard Group’s Target Retirement 2045 Fund, for people retiring in 40 years, had 87 percent of its assets in stocks at the end of 2005, 12.4 percent in bonds and the rest in cash, for example.

But the company’s Target Retirement 2015 fund, for people retiring in just nine years, had only 46 percent in stocks, with 54 percent in bonds.

Over time, the 2045 fund will change its holdings to match the mix in the 2015 fund, and the 2015 fund will put more of its assets into bonds.

This kind of periodic adjustment is exactly what investors ought to do, and these funds serve those who don’t have the time, interest or knowledge to do it on their own.

“It’s a challenging thing to do, and probably can be done a little more tax-efficiently by the mutual fund than by the investor who has fewer holdings,” says Russel Kinnel, Morningstar’s director of fund research.

There are things to watch for.

Kinnel notes that investors should watch out for fees. In some cases, there’s a double layer – those charged by the funds themselves, plus those of the funds they invest in. Morningstar says fees average a modest 0.79 percent, though some charge much more – while some charge only a quarter of that.

Another potential problem: The fund’s asset allocations may not fit your needs, even if your target date matches the fund’s.

Consider the Vanguard 2015 fund. An investor planning to retire in nine years who expects this fund to supply the bulk of his retirement funding might well want to start playing it safe now, and be happy with only 46 percent of his holdings in stocks.

But suppose you’re expecting a pension and Social Security to keep you going in your early retirement years, and that you’re counting on this fund to pay for the later years, after inflation has eaten away at your pension income. If you won’t need this money for 20 or 30 years, you probably should have a bigger allocation in stocks to be sure of enough growth to outpace inflation.

Bottom line: Target-date funds can be a good investment. But you’ve got to do some rigorous long-term planning before you buy.