When was the American "golden age"?
This is, of course, a matter of opinion. To many people, the 1950s were a golden age for the United States although many (mostly minorities and women) would not look at this time as such a golden age.
To many Americans, the 1950s were a golden age because the United States dominated the world economy. This meant that standards of living were rising in the US. Just as importantly, it meant that standards of living in the US were higher than in all other countries.
In addition, this was a time when there was a dominant culture in the US whose dominance was not really challenged. People in this dominant (white, patriarchal) culture felt that the country ran the way they wanted it to. This was a very good feeling.
In short, this was a time when Americans were on top of the world and when white Americans (particularly men) did not feel threatened by any other groups. For this reason, it is seen by many as a golden age.