In November of 2007, there were 121,875,000 full time employees. Now there are 119,287,000.

Fulltime employment is 2,588,000 below the 2007 peak. Meanwhile working age population, 16 and older has gone up from 232,939,000 to 248,446,000. That's an increase of 15,507,000.
 

Simply put, the working-age population has gone up by approximately 15.5 million while fulltime employment has declined by 2.5 million.

Some of this is related to boomer demographics and retirement. A lot of it isn't.
It takes a detailed look at the number of people in each age group and how those age groups have shifted over time to normalize the data. Tim Wallace and I explored this idea once before, for the time period 2010-2013.