Earlier quoted context omitted.
But is it inequality from one person to another, or just unequal life stages for the same individual? You could imagine a college student from a middle-class background earning very little from 20-30, then being middle-income from 30-40, then upper-income from 40-65, then retired until 85. Their independent life (20+) is 65 years, of which 30 are low-income, 10 middle-income, and 25 high income.
Valid point, but I believe that you mean that 10 are low-income.
Another consideration is household size. Before, retirees may have been more likely to live with their children, but now they live more independently.