I don't really think that this is a fair representation. Considering the baby boom, in order for the 2000 data to match the 1900 data, the generations after it would have been off the charts.
If you look at the dashboard link, hovering over the graphs will reveal each point's information. It's disorienting because the percentages are with respect to their year's total population, and the population is much larger now than it was in 1900. By representing things as "percent of total" we can see the stark differences in our demography.
Gotta love that baby boom
This visualization is more about the lack of children being born between 1970 and 2000, which is what the seemingly missing chunk is displaying; this is also notable through the shift of the median age, which is by definition an "aging population". The baby boomers aren't very well represented in this visualization.