Originally Posted by mhays
In many cities, things started to go northward by the early 80s. New York bottomed out in the 70s. San Francisco I'd guess about the same.
New York, San Francisco, Chicago, Philadelphia, and Boston were all seeing inner city gentrification (specifically, growth of the yuppie demographic group choosing to live in and invest in core neighborhoods) as early as the 1950s. The trends toward gentrification in these cities were absolutely unmistakable by the 1970
I see current urban trends as largely driven by demographics and economics, i.e. the relative and absolute sizes of particular age cohorts, their opportunities for jobs and higher education when they reach their formative years, and their proclivity towards raising families earlier or later in life. To a lesser extent, the policy/political environment related to housing in the immediate post-WWII years (discussed further below).
The current crop of "Millenials" are a very large age cohort (much larger than the "Gen X"), who have generally reached adulthood at a time with relatively poorer job/career prospects necessitating spending more time in higher education and deferring the formation of families. Additionally, the Millenials were born, and have lived through, a time of continuous high immigration rates to the US, with international immigration often inherently favoring population migration into, or at least through, the support networks inherent to denser cities.
In contrast to all of the above, the suburbanization trend of the 1950s-1970s occurred at a time with (1) plentiful job opportunities for even the uneducated among the Greatest Generation and (2) very restricted international immigration.
Also, very simply, urban housing stock as of the end of WWII was in terrible shape and very cramped because of the almost complete lack of residential construction and deferred maintenance over the prior 15 years due to the Depression and war rationing. As widely documented, federal housing policy very strongly favored meeting the demand for a more modern housing supply with suburban rather than urban construction. Those policies, and significant federal funding for support, were in full force from the late 1940s through the late 1960s or early 1970s, so it's also natural that their effects would linger for a few decades thereafter.