On average, Americans ages 21 to 42 with at least $3 million in investable assets hold only 25% of their assets in stocks or stock funds, according to a recent survey from Bank of America Private Bank. However, wealthy investors 43 and up hold an average of 55% of their assets in stocks.

Why the disconnect? Although advisors generally recommend that younger investors hold the vast majority of their investments in stocks, wealthy young people are skeptical that traditional investments can get the job done.

In fact, 3 in 4 say it's not possible to achieve above-average returns with stocks and bonds, according to the survey.

As a result, they're opting for alternative investments, such as real estate, private equity and cryptocurrency, to fill out their portfolios.

However, it's not a strategy you'd be wise to emulate, financial pros say.

"While no one has a crystal ball, I would also say that younger retail investors are wrong in thinking stocks will not lead to above-average long-term returns," says Kevin Brady, a certified financial planner at Wealthspire Advisors in New York City.

Here's why he and other pros say you'd be foolish to invest like the jet set.

Young rich investors may 'mistake success with expertise'

Why are young, affluent investors pulling away from stocks? It could be that there are newer, fresher ideas out there, says Ken Shepard, head of investments at Bank of America Private Bank.

Younger people "have lived during a period of incredible innovation. What's modern and new and embraced today could tomorrow be quickly discarded," he says. "This generation has become accustomed to adopting new ideas and ways of doing things."

Plus, he says, the younger generation of investors have now lived through, in some cases, multiple financial crises. "It's created a little more skepticism around the benefits of the equity market."

Meanwhile, plenty of alternative investments have made a lot of people a lot of money over the past few years. But be careful basing your investment choices on recent returns.

"Inexperienced investors — [including] the high net worth young investors who may have made a lot of money with stock options or crypto the past decade — tend to mistake success with expertise," says George Gagliardi, a CFP with Coromandel Wealth Management in Lexington, Massachusetts.

In other words, those who think alternative investments will deliver above-average results over the course of their lifetime may be ignoring that they benefitted from some serendipitous timing with their investments. Or, as Gagliardi puts it: "plain old luck."

Why stocks are still the best option for young investors

Here's the thing about a lot of the investments young millionaires own: Everyday people likely don't have enough money to buy them. "Things like private equity, hedge funds, direct-owned real estate and material amounts of crypto are examples, and the minimum investments for entry are often prohibitive," says Brady.

Even if you have enough money to get in the door, there's little to suggest you'd be better off. For one thing, these investments often come with a high level of risk that everyday investors may not be equipped to handle.

You needn't look further than recent turmoil in the crypto market to see how fortunes can be rapidly lost on alternative investments. Even something seemingly steady, like owning rental properties, can hurt your finances if things go wrong.

"Sure, rents are income, but that's not guaranteed," says Nicholas Bunio, a CFP in Downingtown, Pennsylvania. "And I've seen it with my own clients where they get way in over their head in real estate, ultimately leading to lost money, which set them back years."

Plus, many investments favored by the wealthy come with high fees, which can be a major drag on returns.

Take it from Warren Buffett: "When trillions of dollars are managed by Wall Streeters charging high fees, it will usually be the managers who reap outsized profits, not the clients. Both large and small investors should stick with low-cost index funds."

In other words, no need to get fancy. Financial pros recommend stocks to long-term investors because they have a proven track record of delivering returns. They're cheap, accessible, easy-to-diversify and don't need to be managed by an expensive institution.

You're not guaranteed to get the "above average" returns wealthy young investors are skeptical of if you invest in stocks. But average, depending on how you measure it, may be more than good enough. Since the end of 1945, the S&P 500 has logged an average annual total return of 10.1%.

If you started investing at 22 with $1,000 and deposited $100 per month until you retired at 67, earning an "average" return on your money would leave you with more than $1.1 million, according to Make It's compound interest calculator.

Past performance is no guarantee of future results. But if historical trends hold, stocks give you the chance to rack up compounding returns without having to pay a manager.

"Nothing is guaranteed, but stocks, especially if accessed through a low-cost index fund or ETF, have a long historical track record and actual value to back them up," says Brady. "The same cannot necessarily be said for alternative investments."

Want to earn more and work less? Register for the free CNBC Make It: Your Money virtual event on Dec. 13 at 12 p.m. ET to learn from money masters like Kevin O'Leary how you can increase your earning power.

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