'been meaning to get back to this. I could not recall where I'd read about $500k net worth at age 55...and, fool.com is certainly not source data, they got it from somewhere else.
After a while, I found the Federal Reserve's survey, which they call the Survey of Consumer Finances, aka SCF. They don't have the 55-64 grouping reported by Fool, instead they have it broken down by age.
For famlies headed by a 55 year old, in 2013, they reported a net worth median of $193,750. At the upper end of the Fool's stated range, age 64, the net worth in 2013 is reported by the Fed to be $300,900, again in 2013.
The Fed report methodology description says that the net worths given include savings accounts, 401k, IRA, investment accounts - but not the value of any company pension, if they have one, nor the NPV of social security or other government programs. Plus, equity in only the principal home, but not other homes if they own any. So, there's bias here for the useful value to be on the low side, but not by a lot.
It is worth noting how rapidly the Fed reports net worth rising by age - doubling in ten years is no mean feat. I found the orignal source of my misinformation: a vaguely-worded article in the AARP bulletin of about 16 months ago. This report did include estimated value of employee-sponsored retirement plans, as well as estimates of secondary homes.
The purpose of the AARP bulletin wasn't so much accuracy, but to point out an intereting trend - people's financial success seems to shoot up about when they hit 45 to 60 years old. The primary reason: the kids are out of college!
Gazing over the Fed report, then, it's not surprising to see the spread of hi/low getting very wide at age 45 to 60, but being narrower above and below that range. Below 45, most families still have children in college, above 60, few of them do.