"Median age of the U.S. population 1960–2015" was 28.1 in 1970 median age "Median age of the U.S. population 1960–2015" "Median age of the U.S. population 1960–2015" was 28.1 in 1970 was 28.1 in 1970 median age the U.S. population is getting older the median age "Median age of the U.S. population 1960–2015" "Median age of the U.S. population 1960-2015" two years