Nearly three-quarters of Americans get stressed out or anxious when they think about retirement saving and investing.

Americans are highly anxious about their retirement prospects, a report from the National Institute on Retirement Security found.

TDFs' explosive growth masks the fact that they are still in their infancy, said Rich Weiss of American Century Investments.

Fidelity Investments is ramping up its small presence in the rapidly growing exchange-traded fund business by expanding a 3-year-old partnership with BlackRock Inc.'s iShares unit, the largest ETF provider.