Saw a print edition of Barron’s, December 2, 2019, page 14.
It said that in 1989, when Boomers’ average age was 35, they held 21% of the wealth in the US. But in 2023, when Millennials’ average age will be 35, they will control 3% of the wealth in the US.
That’s because by 2023, the average 69 y/o Boomers wouldn’t have died yet, so their kids haven’t inherited the wealth.
^^ That’s a strange argument. In 1989, the average parents of the boomers hadn’t died yet either…
I think it’s tied to overall trends as to the concentration of wealth in the US.
Look at figure 2 here - https://equitablegrowth.org/the-distribution-of-wealth-in-the-united-states-and-implications-for-a-net-worth-tax/
Or check the charts in this article: https://www.washingtonpost.com/us-policy/2019/02/08/wealth-concentration-returning-levels-last-seen-during-roaring-twenties-according-new-research/
It’s a sign of the times, not something tied to age – we boomers at age 65 also don’t account for as high a share of wealth as our parents did when they were our age.
OK Boomer. 
I believe the source for the quote is https://www.federalreserve.gov/releases/z1/dataviz/dfa/distribute/table/#quarter:0;series:Net%20worth;demographic:generation;population:all;units:shares . The link provides additional comparisons such as below.
Share of Wealth Change from 1989 to 2019
Age Under 40 – Decreased from 13% to 7%
Age 40-55 – Decreased from 32% to 21%
Age 55-70 – Increased from 35% to 45%
Age >70 – Increased from 19% to 28%
Boomers in 1989 had $1.9T Real Estate, Millennials in 2019 have $0.2T
*Assets - Liabilities, Adjusted to 2019 Dollars
There are many factors that contribute to these effect. The increasing wealth disparity is the primary factor, but a larger portion of the population in higher age groups is also relevant.
I looked at the table. The wealth disparity was greater in 1989 between the Silent generation (those born before 1946) and the Boomer generation than is the disparity between the Boomer generation and the younger generations today.