The data this study is built on, and its methodology (not described in this report) are inflicted with a few problems.
For starters, this is data gathered in the 99/00 time frame at the height of the bull market. I would hazard a guess that data from 97 or 03 would show a markedly different result.
Next, the methodology for gathering data for this (from my recollection, it'll be another cup of coffee before I fee like looking for it and re-reading it) was that they invited a fairly limited number of random households to bare their financial selves to the panel. If any declined, additional invitations were not extended.
Thirdly, these numbers are not audited; they're what the families SAID their assets and debts were.
Lastly, they do not say exactly how many families were specifically audited, only how many were invited. Even the invitation sample is not statistically valid for the group size.
So what you have is probably misreported and inaccurate information from a statistically invalid sample of people who wanted to brag at a time when net worth was at an all time high.
With regards to the original question, while I doubt that anyone can give a really valid answer by age group, its safe to say that cut-throats analysis is accurate. Most people in most age ranges dont have a pot to piss in, unless the pot was bought on a credit card or a HELOC. In fact, I'd hazard that a lot of the seemingly well to do families you know are living on a mountain of debt, rather than a positive net worth.