I've heard it's dropping because the Baby Boomers are retiring. I've also heard it's actually higher in that group and it's younger workers with the lower LFPR
Total:
Interestingly it corresponds to the rise and subsequent fall in women entering and leaving the labor force as participation rate for men has been declining steadily since the 1950's.
Here is the data by gender:
Women:
Men:
Now the question is, how much of the decline since the peak in the 1990's is "structural" versus "cyclical."
Participation rate in 55+ year olds had been increasing until recently and appears to have rolled over and started declining as baby Boomers start to retire:
Meanwhile participation rate for 20-24 year olds has been declining but appears to have flattened out and will likely start to rise along with the Millennial population boom entrance into the workforce.
Keep in mind that there are more 24 year olds in the US than any other age group.
while the workforce overall has shrunk nearly 10 million since 2009, the cohort of workers in the labor force ages 55 to 64 has actually increased over that same period, with many delaying retirement due to poor economic conditions.
over two-thirds of all labor force dropouts since that time have been under the age of 55.
http://www.freerepublic.com/focus/f-news/3208166/posts