The index, which goes back to 1985, measures the amount of income you need to enter the top 25 percent of households in the United States. Back then, the income threshold was lower ($30,928) and yields were higher than the historical average. [...] I calculate how much you'd need at a 4 percent withdrawal rate. For retirees, I add how much you'd need to provide the same income if Social Security kicked in 40 percent. Slice it whatever way you like, you need a lot of investment money if you want to live on interest and dividends. Between 2002 and 2012, the labor force participation rate of those 65 and older rose from 13.2 percent to 18.5 percent. The participation rate increase may be part of the explanation for why things are so tough for other workers, in particular the youngest. Labor force participation rates fell for all workers as a group, but the biggest decline in participation was for workers ages 16 to 24.