In the current system, people 65 and over are eligible for Medicare, which Obama has said he would keep for now.
Romney has proposed raising the eligibility age for Medicare beneficiaries from 65 to 67 in 2022, then increasing it by a month each year after that. In the long run, he would index eligibility levels to "longevity." Ryan's budget plan proposes raising Medicare eligibility age by two months a year starting in 2023, until it reaches 67 by 2034.
Many others looking to keep Medicare solvent have also proposed raising the age of eligibility.
The Congressional Budget Office estimates that raising the minimum age from 65 to 67 would reduce annual federal spending by 5 percent. But it would also result in higher premiums and out-of-pocket costs for seniors who would lose access to Medicare.
The Affordable Care Act is projected to reduce Medicare spending by $716 billion over the next 10 years. These reductions, as detailed by Washington Post's Wonkblog, will come mostly from reducing payments to hospitals, nursing homes and private health care providers.
"The ACA grows the trust fund by giving more general revenue to the Treasury, which then gives the trust fund bonds. But it then uses the money from those bonds to expand coverage for low- and middle-income people," explains Dylan Matthews on Washington Post's Wonkblog.
Romney hasn't really come up with a solid answer: he previously said he would restore the $716 billion savings that the health care law imposes. Per this New York Times story, the American Institutes for Research calculates this would increase premiums and co-payments for Medicare beneficiaries by $342 a year on average over the next 10 years.
For more on where the candidates stand on the $716 billion, the private health policy Commonwealth Fund offers this helpful explanation.
Caps on Spending
Both Obama and Ryan have set an identical target rate that would cap Medicare spending at one-half a percentage point above the nation's gross domestic product.