The youth of the housing stock in 1975 was a consequence of the postwar baby boom, which was also a building boom providing a fresh supply of housing for a rapidly expanding and increasingly affluent population. In the following four decades, new housing has been a shrinking share of the total housing stock. Because of the aging of the housing stock, it will be many more years before new building technologies are a part of the average American home. But there's a silver lining—growing opportunities for renovation and building upgrades.
Occupied housing units: median year built (and age of average structure)
2015: 1976 (39 years)
2005: 1973 (32 years)
1995: 1967 (28 years)
1985: 1962 (23 years)
1975: 1957 (18 years)
Source: Census Bureau, American Housing Survey