Maybe it's because they're young and more likely than older workers to hop from job to job, but millennials are leaving behind a trail of old 401(k)s in their wake.

While that's not always a big mistake, in some cases it leads young investors to miss out on other opportunities to grow their retirement nest egg.

Among mass affluent investors with at least $50,000 in investable assets, 62 percent of 18 to 24-year-olds had at least one 401(k) from a former employer in 2017, A.T. Kearney's 2017 Future of Advice study shows. The average size of those accounts was $33,000.

Among older millennials – those ages 25 to 34 – 59 percent had at least one "orphan" 401(k). Those accounts averaged $47,000.

[See: 11 Steps to Make a Million With Your 401(k) .]

Many young investors had more than one of these accounts. In contrast, 41 percent of investors of all ages had at least one orphan 401(k) account in 2017.

The results aren't particularly surprising, says Monica Gabel, principal at A.T. Kearney's Financial Institutions Group.

"Millennials are more mobile. They switch jobs way more often than older investors," she says.

The other factor: the process to rollover a 401(k) into another 401(k) or individual retirement account , is not always digitalized depending on the provider, she adds.

"There's a lot of things that make this a tedious process, and millennials tend to be very self-service and digital," Gabel says. "The process is not easy to execute, so they'll forget for now and deal with it later."

While it's not always a mistake to leave a 401(k) with a former employer, it's definitely not wise to forget or neglect an old account. And that's the danger with orphan 401(k)s.

"The more you wait, the easier it is to forget," Gabel says. "And sometimes, the amount they have saved might not warrant the effort it takes to move it. The smaller amount, the more likely they are to forget about it."

Reshell Smith, a financial planner in Orlando, Florida, recently helped one of her clients rollover three orphan 401(k)s into one, consolidated account. The client had forgotten about one of the accounts for years, partly because it had only $296 in it. The money was sitting in cash and hadn't benefited from the run-up in stock prices over the last few years.

"A lot of times, when you leave orphaned 401(k)s, you forget how they're invested," she said. "You may have money sitting in cash, which means you've missed opportunities to have that money working for you in the stock market."

The other danger: If investors don't stay on top of their old 401(k)s, they may fail to miss any key changes to the plan's options, which could result in higher fees or different allocations than you had anticipated.

"You want to make sure that you keep track of not only what you have and where, but where that money is invested," Gabel says. "Make sure you review it on a regular basis to make sure you're optimizing the allocations and options."

[See: 7 Things You Need to Understand About Your 401(k) .]

If you do decide to roll over an old 401(k), there are several options. You can consolidate it with your new employer's 401(k) or roll it over into an IRA. Or, if you're self-employed, you can open a solo 401(k) and roll it over into that account. Solo 401(k)s allow the participant to act as both the employee and the employer, and therefore save a higher amount than in traditional employer-sponsored accounts.

If you conduct the rollover in a year when you have low income – for example if you're starting a business and have yet to earn a profit – it can also be a great time to rollover an old 401(k) into a Roth 401(k) . That rollover will require you to pay a one-time tax bill on the account, but from that point forward, any funds in the Roth 401(k) can grow tax-free, saving you from larger payments to Uncle Sam in retirement.

Regardless of where you decide to direct the rollover, be careful about the process. Make sure it's a direct rollover from one retirement account to another, says Ash Foster, a financial planner in Houston. In direct rollovers, the old 401(k) provider connects directly with the new plan's provider and transfers the money from firm to firm.

In contrast, in an indirect rollover, the 401(k) custodian may send a check back to the plan participant. In that case, if the participant fails to invest the money in a new qualified retirement account within the next 60 days, he or she could face "nasty tax consequences," Foster says.

When does it make sense to maintain old 401(k)s? If the old provider offers a large menu of investment options with low fees, it may be easier to just keep the old account, says Matthew Ramos , a financial planner in Washington, D.C.

It's also worth noting that 401(k)s can offer more legal protection than IRAs, he says. Whereas 401(k)s are protected by federal law from both creditors and judgments, IRAs can offer fewer protections, depending on your state laws.

With all of these orphan 401(k)s out there, millennials present a lucrative target for the financial services industry. Overall, mass affluent investors ages 18 to 34 have $120 billion in orphan accounts, about 48 percent of which A.T. Kearney estimates will be moved into new accounts in the next year. Millennials are more likely than older investors to consider robo advisors to manage those assets, and they're less loyal to any one financial services brand, Gabel says.

[Read: 5 of the Best Stocks to Buy for April .]

That's a big opportunity for financial services firms to attract those assets.

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