Median U.S. home age drops, now built in 1980, 4 years newer than a decade ago.
86% of cities saw renewal, driven by new builds, migration, and urban expansion.
Sunbelt and suburban areas surged, fueling younger housing stock across the nation.
Redevelopment, gentrification, and disasters reshaped older neighborhoods with newer homes.
Every state saw progress, though pace varied widely from modest to dramatic.