Relative affluence or poverty is less a matter of fixed groups than of lifespan. The young are poor, the middle-aged are gaining traction, the near-retired have peaked, and old age is a long decline - in wealth and income as well as health. When we're young, we're getting educated and sampling jobs - a process that can extend into our 30s. Eventually we settle on some career trajectory and start accumulating skills and money. Most of us will be home owners by age 40. Since the biggest source of wealth in the US is home equity, the longer we own homes, the wealthier we're likely to be. Over half of us will also invest in stocks and mutual funds - the value of which peaks when we reach our late 60s. Hence, the association between age and wealth.
As for the association between age and income, this chart pretty much says it all:
Of course, there are plenty of people who are chronically poor. Not everyone is able to improve their circumstances as they get older. The less fortunate and vulnerable need help - especially single moms. But most of us will reach at least the middle class at some point in our lives.
Next: How many?
Investment Company Institute/2013 Investment Company Fact Book
US Census Bureau: Survey of Income and Program Participation, 2008 Panel