Every year, economist Mark Perry draws on Census Bureau reports to paint of picture of the demographics of inequality. Looking at 2015 data, he constructed the following table:
Household demographics, including the average number of earners per household and the marital status, age, and education of householders are all very highly correlated with household income. Specifically, high-income households have a greater average number of income-earners than households in lower-income quintiles, and individuals in high income households are far more likely than individuals in low-income households to be well-educated, married, working full-time, and in their prime earning years. In contrast, individuals in lower-income households are far more likely than their counterparts in higher-income households to be less-educated, working part-time, either very young (under 35 years) or very old (over 65 years), and living in single-parent households.
The good news is that the key demographic factors that explain differences in household income are not fixed over our lifetimes and are largely under our control (e.g. staying in school and graduating, getting and staying married, etc.), which means that individuals and households are not destined to remain in a single income quintile forever. Fortunately, studies that track people over time indicate that individuals and households move up and down the income quintiles over their lifetimes, as the key demographic variables highlighted above change, see C[arpe] D[iem] posts here, here and here.