The way I read it is that you look at the median wealth of a given cohort as of 1989, and that is the baseline. From there, you look at how the median wealth of that cohort has increased or decreased every three years.
Its not that everyone's wealth was the same in 1989, but that since that time, whatever the wealth of a given cohort was has diverged dramatically.
It is behind a paywall, but there's a lot of other interesting data as well on generational differences in wages and wealth.