WASHINGTON (Reuters) - One in five older working Americans say they will never retire, whether out of financial need or to stay active and engaged, according to a study released on Monday.

Those who plan to keep working include a fifth of older workers who will be able to collect retirement benefits and roughly the same number of workers who have no such benefits like pensions, according to the survey of 2,719 U.S. residents between 55 and 70.

The study was conducted for the MetLife Mature Market Institute, part of insurance and financial group MetLife Inc.

Also among those who don't plan to retire are about 18 percent of respondents who retired and then re-entered the workforce, it said.

They tend to be people who experience a "year of euphoria" immediately after retirement but grow bored, said Sandra Timmerman, director of the Mature Market Institute.

"You begin to look at your life and think, 'What is life all about?" she said. "It's not one endless game of golf or a long vacation. Time becomes a more precious commodity."

The oldest workers were most likely to keep working to stay active and engaged and least likely to keep working out of financial necessity, the survey showed.

But 72 percent of the workers in the study aged 55 to 59 said they need the income to live on.

"Older workers are not as motivated by income but by meaning," said David DeLong, a consultant who helped conduct the study.

"There's a lot of desire to step off the stress treadmill," he said. "As you get older, money becomes less of a driver, even if it means cutting back on the lifestyle."