Large U.S. cities lost tens of thousands of millennial and younger Gen X residents last year, according to Census figures released Thursday that offer fresh signs of cooling urban growth.

Cities with more than a half million people collectively lost almost 27,000 residents age 25 to 39 in 2018, according to a Wall Street Journal analysis of the figures. It was the fourth consecutive year that big cities saw this population of young adults shrink. New York, Chicago, Houston, San Francisco, Las Vegas, Washington and Portland,...