Investors are realizing what advisors have long known: retirement planning isn't easy.

About two in three investors (64%) between the ages of 21 and 64 who are not seeking to retire within five years said they have difficulty with retirement planning, up from 54% who said so just two years ago, according to a new study from Hearts & Wallets, a research firm based in Hingham, Mass.

The increased difficulty is linked to investors' inability to identify what year they might stop working full-time, said Laura Varas, a partner and co-founder of Hearts & Wallets.

"Investors aren't sure, as they put it, when their last job will be," Varas said. Not knowing—and not being in control of—the timing when full-time work stops is a huge shift from the halcyon days when people could count on retiring at 65, she added.

Apart from the big "when" question, investors with more than five years to retirement struggled with estimating the required minimum withdrawals from their retirement accounts (57%), deciding where to put their savings (54%) and getting started saving and investing (51%), the research found.

The growing realization of the complexity of retirement planning creates opportunities for advisors, noted Chris Brown, a principal and co-founder of Hearts & Wallets. "More than 50% of all U.S. households currently aren't getting the help to be prepared for whatever the future brings," he said.

Even among sophisticated investors, the need for professional advice is quite strong. According to the research, 45% of investors with $500,000 to $2 million in investible assets reported having difficulty with retirement planning. More than one in four (27%) with more than $2 million in assets found the task difficult. 

"Retirement saving is much more than regularly sending a portion of one's paycheck to a 401(k) provider, but how many young Americans realize this?" Brown said.

The study is based on a survey of more than 5,000 U.S. households conducted in June of 2014.

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