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If you are in your 20s or early 30s, and you ask around for retirement advice, you will hear two things:

1. Put as much as you possibly can, as soon as you can, into a 401(k) or Individual Retirement Account.

2. Put nearly all of it into equities.

There's a lot of common sense to this. Saving early means you can take maximum advantage of the compounding of interest. And your youth makes it easier for you to bear the added risk of equities.

But life is more complicated than these simple intuitions suggest. Here's a troubling data point: According to a Fidelity survey of 401(k) plan participants, 44% of job changers in their 20s cashed out all or part of their money, despite being hit with taxes and penalties. Switchers in their 30s were only a bit more conservative, with 38% cashing out.

You really don't want to do this. But let's get beyond the usual scolding. The reality that so many people are cashing out is also telling us something. Maybe a 401(k) loaded with stocks isn't the best savings tool for some young people.

The conventional 401(k) advice—which is enshrined in the popular "target-date" mutual funds that put 90% of young savers' portfolios in stocks—imagines twentysomethings as the ideal buy-and-hold investors, as close as individuals can get to something like the famous, swashbuckling Yale University endowment fund. Young people have very long time horizons and no need to sell holdings for current income, the thinking goes, so why not accept the possibility of some (violently) bad years in order to stretch for higher return? But on a moment's reflection on what life is actually like in your 20s, you see that many young people are already navigating a fair amount of economic risk.

Take career risk. On the plus side, when you're young you have more years of earnings ahead of you than behind you, and that's a valuable asset to have. Then again, you also face a lot of uncertainty about how big those earnings will be. If you are just gaining a foothold in your career, getting laid off or fired from your current job might be a short-term paycheck interruption—or it could be the reversal that sets you on a permanently lower-earning track. You may also be financially vulnerable if you still have high-interest debts to settle, a new mortgage that hasn't had time to build up equity, or low cash reserves to get your through a bad spell.

This is why Micheal Kitces, a financial planner at Pinnacle Advisory Group in Columbia, Md., tells me he doesn't encourage people in their 20s to focus on building their investment portfolio. You almost never hear that kind of thing from a planner, so let me clarify that he's not saying you should spend to your heart's content. (Kitces is in fact a bit stern on one point: He thinks many young professionals spend too much on housing.) He's talking about priorities. For one thing, you need to build up that boring cash cushion. Without it, you are more likely to be one of those people who has to cash out the 401(k) after a job change.

Even before that's done, you'll still want to aim to put enough in a 401(k) to max out the matching contributions from your employer, if that's on the table. (Typically, that's 6% of salary.) So maybe all or most of that goes in stocks? An attention-getting new brief from the investment strategists Research Affiliates argues "no"—that instead of putting new savers into a 90%-equities target date fund, 401(k) plans should get people going with lower-risk "starter portfolios."

I'm not sold on all of RA's argument, which drives toward a proposal that 401(k)s should include unusual funds like the ones RA happens to help manage. But CEO Rob Arnott and his coauthor Lilian Wu offer a lot to chew on. They make two big points about young people and risk. One's just intuitive: If you have little experience as an investor and quickly get your hat handed to you in a bear market, you could be so scarred from the experience that you get out of stocks and never come back. At least until the next bull market makes it irresistible.

The other is that 401(k) plan designers should accept the fact—all the advice and penalties notwithstanding—that many young people do cash them out like rainy-day funds when they lose their jobs. And so the starter funds should have a bigger cushion of lower-risk assets. That's especially important given that recessions and layoffs often come after big market drops, so the people cashing out may well be selling stocks at exactly the wrong moment, and from severely depleted portfolios.

RA thinks a portfolio for new savers should be made up of just one third "mainstream" stocks, with another third in traditional bonds and the last third in what it calls "diversifying inflation hedges." That last bit could include inflation protected Treasuries (or TIPS), but also junk bonds, emerging markets investments, real estate, and low-volatility stocks. Whatever the virtues of those investments, it seems to me that a starter portfolio should be easy to explain to a starting investor. "Diversifying inflation hedges" doesn't sound like that.

But the insight that new investors might not be immediately prepared for full-tilt equity-market risk is valuable. Many 401(k) plans automatically default young savers into stock-heavy target date funds, but they could just as easily start with a more-traditional balanced fund, which holds a steady 60% in stocks and 40% in bonds. Perhaps higher risk strategies should be left as a conscious choice, for people who not only have a lot of time, but also a bit more market knowledge and a stable financial picture outside of their 401(k).

The trouble is, most 401(k) plans don't know much about an individual saver besides their age. The 401(k) is a blunt, flawed tool, and just putting different kinds of mutual funds inside of it isn't going to solve all of the difficulties people run into when trying to save for the future. Arnott and Wu's proposal doesn't do anything about the fact that using a 401(k) for rainy days means paying steep penalties. And it doesn't help people build up the cash reserves outside their retirement plans that they'd need to avoid that.

As boomers head into retirement, we've all become very aware of the importance of getting people to prepare for life after 65. But millennials also need better ideas to help get them safely (financially speaking) to 35.