Basically where the histogram peaks are, which doesn't tell you much about the rest of the distribution within each state. The first commenter makes a good point:
It would be much more informative (and less deceptive) to see a graphic for each state that showed percentage across all decades, including the 1600--1800s. The East has houses still in use from the colonial era, and certainly many from the late 1800s.
1950-1959: California, Connecticut, Illinois, Kansas, Michigan, Missouri, New Jersey, New York, Ohio, Pennsylvania, Rhode Island, Wisconsin