That's not by definition, although it is how social security works.
Retirees have assets but not income (or they have low income). Younger people are the other way round. So depending on how governments use income taxes vs sales taxes vs property taxes it changes who pays for things.
California is the worst about this because of Prop 13, which basically means if you don't move then your property tax is much lower than it should be and newer residents pay for you.
Who else would pay for retirees, if not the non-retirees?
Most of their assets are paper value, because they are located in stocks, their only home (and they have to live somewhere), etc… that if sold en masse would simply get pennies on the dollar or require expenditures elsewhere.
