The Census has data showing what percent of married couples have two incomes or just one income.
First of all lets look at all married couples:
This shows a fairly large minority of 17% of couples where neither spouse works. This data does not exclude retired individuals so I assume the majority of those 17% are senior citizens.
Now lets look at married couples who have children :
Since these are couples with children we can assume that almost all of them are not of retirement age.