Source: Mark R. Rank & Thomas A. Hirschl, 2001. "Rags or Riches? Estimating the Probabilities of Poverty and Affluence across the Adult American Life Span," Social Science Quarterly, The Southwestern Social Science Association, vol. 82(4), pages 651-669.
From the abstract:
Between the ages of 25 and 75, 51.1 percent of Americans will spend at least one year below the poverty line, 51.0 percent will encounter a year of affluence, while only 21.0 percent of Americans will avoid either of these economic extremes.The research is based on an analysis of the Panel Study of Income Dynamics (PSID). Poverty is defined using the US Census Bureau criteria. Affluence is defined as ten times the poverty levels. For more on income volatility, see here.