Because people saying that are doing accounting with money, not productivity.
People need three things to live in retirement. Food, water, shelter[1], and some degree of medical care. We are not in a demographic collapse situation where society cannot be productive enough to fail to provide the first three to the retired. The latter is the only thing in danger of seeing some cuts.
[1] Housing isn't expensive because building four walls and a roof is expensive. Housing is expensive because there's no upward bound on the cost of a square foot of land in a city.
What if the votes of each age group were weighted to account for the size of that group, e.g. making the votes of 18-20 have the same weight as the votes of 40-42, even if there may be more people in the 40-42 group.
This would prevent an aging population from giving too many benefits to older age groups, and would encourage policies that consider long term impacts more.
Because the principle of "one person, one vote" is widely (and correctly IMO) accepted.
It’s unclear that an age-based distribution instead of geographic distribution would be worse. But it would probably be unrealistic to achieve without basically completely upending existing political structures.
The US is one country and disparate voting power between states(mainly in the Senate, to a lesser extent with the electoral college) is a real problem. I see little reason to make it worse. Although it's useful to rhetoric out how little sense the current system makes
What is actually practiced is 'One collection of zip codes, one vote.'
My proposal is: Multiply the number of house reps and senators by ten (for instance). Each representative now represents a district / state as well as a decade of life (for instance). When voting for an age-graded role, your vote only counts towards the decade of life that you're currently in. You can only run for a position that matches your age range.
This would have a number of advantages: Disrupt two-party dynamics. Increase granularity of representation. It's harder for older folks to take younger folks hostage. Younger folks will feel more enthusiastic about voting for someone who actually represents them.
Probably, you'd want to fiddle with the numbers based on age demographics. 7-year intervals is probably the right number, with a big bucket for people over a certain age. (So, 18, 25, 32, 39, 46, 53, 60, 67, 74+) might be right.
and that's why it'll never happen without a revolution
The problem is in the short/medium term, figuring out how to pay for all these checks that politicians have been writing over the last 50 years.
The problem isn't that "the young" have to support "the old", it's that a lot of promises were made on the assumption of indefinite growth.