Obviously Romney/Ryan think that they can defuse negative reaction to voucherizing Medicare by promising to limit the changes to people born after the 1950s. Since it is the over-50 voters who have the heaviest voter turnout, they reason, they will not care if we shortchange the younger people. We can even say that we are doing it "to save the program" for the current recipients.
They are probably wrong about how older people will react (a similar plan for Social Security got a very poor response even among those who "would not be affected"), but if anyone you know is tempted by this promise, tell them that here is what is going on:
HOW TO DESTROY AN ENTITLEMENT
[1] Label it a welfare program that results in successful people supporting those who are failures. In the case of Medicare, this involves always talking about it together with Medicaid and emphasizing that many people have better employer-supplied insurance (ignoring the fact that most of this shrinking group are either union workers or government employees). The GOP has been doing this for years.
[2] Means-test the program, either directly by excluding those who are financially independent or indirectly by making the benefits taxable in some way for such people (as has already been done with Social Security). This helps weaken support for the program even from its most powerful and politically-active recipients. It is always a bit difficult for the GOP to make a "let the rich pay more" argument of this kind with a straight face, but they can usually find some short-sighted Democrats to front for them here. This will be the short-term GOP fall-back position if they get into power but lose some of their troops on outright voucherization.
[3] Exclude a large portion of the population from the program. For Medicare do this based on date of birth, reinforced by the existing exclusion of even those undocumented workers who pay the taxes to support it. These exclusions insure that a growing proportion of the population no longer benefits from the program. When that proportion gets big enough, move on to step 4.
[4] Abandon the program altogether as soon as the next fiscal crisis arrives (or can be manufactured). If this cannot be done all at once, starve the program to death by freezing its funding despite inflation and growing need.
Short form: "If the GOP succeeds now in making it so that most voters will not benefit from Medicare-as-we-know-it, don't you think that in about 10 years they will declare that 'fairness' demands that we apply the same rules to everyone?"