We can argue back and forth about whether it amounts to a subsidy or something else. But if you just look at aggregate cash flows the largest flows from Washington out to rural areas are social security and government paid for medical care (medicare, medicaid, va, ppaca subsidies). The largest flows from the rural areas to Washington are the individual income tax and payroll taxes.
If those are significantly out of line with each other, then the net offset is going to swamp relatively piddly flows like the Connect America Fund or the Rural Utilities Service.
1) I don't think it just so happens that there are more old people in rural areas. It has to do with the respective characteristics of rural and non-rural areas rather than some historical accident.
2) I think even if you excluded those over 65 you would still find net flows as I've described them because rural areas have too many poor people and not enough rich people.
The larger point is that if you look at the United States as a country where we are all in this together, then it makes no difference if the people getting social security disproportionately live in Wyoming and the people making max social security contributions are disproportionately in NY. Individuals might be net contributors or recipients but if the relevant group is Americans than it all sums to zero. Most of the time that's the attitude most Americans adopt. But then when it comes time to try to justify the continued existence of the Senate and the Electoral College all of a sudden it's 1789 again and the States are independent sovereigns that merely turned over a handful of responsibilities to the Federal government. All of a sudden we aren't all Americans after all but instead Wyomingites that want to know why they should be ruled by New Yorkers and Californians.
If those are significantly out of line with each other, then the net offset is going to swamp relatively piddly flows like the Connect America Fund or the Rural Utilities Service.