First, there's almost no relevant metric sorting Earthers that would have an American as the median.
Second, it is absolutely nuts to suggest the average American citizen was better off in the 1950's. 1950's America still had a substantial amount of truly crippling poverty, it had millions of citizens who had almost no chance at that great office job due to their race or gender, and life expectancy at birth was lower by almost a decade.
And yes, that's before you figure the months of salary to buy, say, a refrigerator, that the median wage earner can now afford on a week's pay. A refrigerator, by the way, that will be larger, last longer, and use substantially less energy than the one from 1955.
Or the huge growth in sales of air travel, entertainment, dining... luxuries in the 1950's that are commonplace today.
Yes, access to cheap credit makes it easy to bid up the prices of scarce resources like slots at good colleges or houses in desirable areas, but the average American isn't paying out of pocket much more for those things as a % of their income than they were 70 years ago. And of course the colleges are much nicer places to live and study than they were, and the houses are bigger and better constructed.