It would seem to make sense that as one ages, and doesn't have the resources to take care of his/her own medical needs that he/she would favor Medicare, or some other "socialist" solution over dying sooner. Libertarians are against taxation (excpet for one segment that favors taxes for national defense). Now Medicare resulted from the passage of a bill in Congress. I assume that the overwhelming majority of those who voted for it were looking toward their next election. That is one of the reasons that "term limits" are so important. If Congressman and Senators don't have the option of another term they might be more likely to vote the "national" interest over their own. So things such as Medicare may become less likely in the future with term limits.
Another factor is that the elderly (along with other voting segments) are too lazy to vote. Luckily for those with "conservative" agendas those who most benefit from a "socialist" agenda (the elderly, and the poor) often don't bother to vote. This (not the "Reagan" revolution) is why "conservatives" have done so well since 1980. Of course, the poor are often the most "conservative" (when it comes to social issues) of voting segments. You'll find many "fiscal" conservatives are on their second marriage, and have a mistress on the side. Hopefully, over time democracy will give way to the libertarian ideal, and we can have a life expectancy of around 40-50 "productive" years, and not have to clutter the political landscape with our desires for immortality.