Are you suggesting that the bottom 90% of households are predominately headed by under-40 year-olds?
According to payscale.com, among those with college degrees or higher, pay for men stops growing about age 48; for women, about 39. (See http://www.payscale.com/gender-lifetime-earnings-gap )
In your defence, there is this: http://www.usnews.com/opinion/blogs/economic-intelligence/20... ; in which Joseph Mason, Moyse/LBA Chair of Banking at the Ourso School of Business at Louisiana State University and a senior fellow at the Wharton School of the University of Pennsylvania, suggests that the demographics of income and wealth inequality will go away with the Baby Boomers. However, that article doesn't go into or cite any real analysis.
A contrary opinion by Phillip Longman in 2008 (at http://newamerica.net/files/Longman-Remarks.pdf) actually goes into more detail on baby boomers and money: Could it really be that a typical household headed by a person in their 50s has disposable income of over $85,000 dollars? ... [Sure] enough, [the Census Bureau’s Current Population Survey] shows average household income numbers for that cohort in the range of $85,000. ... The medium income numbers were much, much lower. For example, in 2006, according the Census Bureau, households headed by a person 45-54 had an average income $85,812. But the medium income of such households was some $20,000 less. The difference between average and medium works out to about 25 percent. ... Income inequality, let alone wealth inequality, in this generation is extremely high. Its average income includes the earnings of Bill Gates and countless other baby boom billionaires, plus a very large new class of McMansion-owning minor millionaires. Because of very high income inequality within the baby boom generation, medium income statistics tell us a much more accurate story than do average income stats about how the “typical” baby boomer lives.
Age could easily explain the apparent "wealth inequality", but not so much in regards to "income inequality". Yet "income inequality" can easily be shown to be a factor towards "wealth inequality". According to Pew Research (see http://www.pewresearch.org/fact-tank/2014/12/17/wealth-gap-u...): The wealth gap between America’s high income group and everyone else has reached record high levels since the economic recovery from the Great Recession of 2007-09, with a clear trajectory of increasing wealth for the upper-income families and no wealth growth for the middle- and lower-income families. In 2013, the median wealth of the nation’s upper-income families ($639,400) was nearly seven times the median wealth of middle-income families ($96,500) - the widest wealth gap seen in 30 years when the Federal Reserve began collecting these data.
My suggestion: Let's stop conflating the issues of "wealth inequality" and "income inequality". The real crux of the matter is in the "income inequality". There's been wage stagnation for decades despite productivity gains. In America anyway, the tax code is rigged against earned income in favor of unearned income. Overall, it is better to be "wealthy" than to be "gainfully employed".