Becoming a millionaire used to be something young Americans dreamed about. Now, according to one survey, it's viewed as a necessity.

That is, if you're a young adult who wants to be able to retire.

A new online survey by the Northwestern Mutual Foundation found that 85% of respondents between 18 and 29 years of age said they needed at least $1 million to retire. Nearly half-45%-said they needed $2 million.

Among those 30 and over, 60% said they needed $1 million and only 27% were in need of $2 million.

The big-dollar figures are not only due to the expected impact of inflation in the decades to come. The survey's authors note that people continue to live longer, with the average lifespan increasing more than 30 years over the past century. The U.S. Census Bureau projects average life expectancy will exceed 79 years by 2015, according to the foundation.

"The phrase 'life goes on' is quickly becoming 'life goes on and on and on,'" said Meridee Maynard, financial literacy expert and senior vice president of Northwestern Mutual. "Young people may spend more time and money in retirement, but fortunately, they also have time to prepare."