I see three potential caveats with this explanation. 1) The author here uses registered births. This chart, which would include immigrants (about 42M individuals, or 13% of the population) looks less dramatic: 2) The PEW data showed that 65 and older had a 24.6% increase, but that age excludes most of the baby boomer population (those 51-64) as defined by the generational graph. The PEW data shows that 45-64 yos had a 2.1% increase and 30-44yos had a 0.1% decrease. 3) Did Social Security really push significantly more middle class baby boomers into the upper middle class ($126,000 to $188,000 year) than it kept from falling into the lower middle class in addition to pulling lower middle class baby boomers into the middle class? Because only then could it be attributed to shrinking the middle class contribution for that generation.