Question: What percent of American adults live in households in the Top 20 percentile of income at some point before they're 60 years old?

Answer: 69.8%

Question: Of those Americans who reach the top 20 percentile of income at some point before the age of 60, how many spend at least 5 years in the top income percentile?

Answer: 46%

The following tables tell the story better. First, the cumulative percentage of American adults experiencing the top 20% of household income by age:

Standard errors in parentheses. Source: Hirschl TA, Rank MR (2015) The Life Course Dynamics of Affluence. PLOS ONE 10(1): e0116370. https://doi.org/10.1371/journal.pone.0116370 https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0116370

Standard errors in parentheses. Source: Hirschl TA, Rank MR (2015) The Life Course Dynamics of Affluence. PLOS ONE 10(1): e0116370. https://doi.org/10.1371/journal.pone.0116370 https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0116370

Second, the number of years those who make it to the top 20% of income stay there:

Standard errors in parentheses. Source: Hirschl TA, Rank MR (2015) The Life Course Dynamics of Affluence. PLOS ONE 10(1): e0116370. https://doi.org/10.1371/journal.pone.0116370 https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0116370

Standard errors in parentheses. Source: Hirschl TA, Rank MR (2015) The Life Course Dynamics of Affluence. PLOS ONE 10(1): e0116370. https://doi.org/10.1371/journal.pone.0116370 https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0116370

In other words, social mobility is alive and well in the US. Yes, some people are stuck at the bottom, but that's a subject for another post.

Reference:

Hirschl TA, Rank MR (2015) The Life Course Dynamics of Affluence. PLOS ONE 10(1): e0116370. https://doi.org/10.1371/journal.pone.0116370  https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0116370