Usual disclaimer: This is my opinion, yada yada yada. I am probably wrong on some (if not most) of this.
The elderly, especially in America, have an incredible amount of sway over national politics. Nearly 1/2 of voters are 50 or older [1]. As such, they generate a lot of ire in the US among the less-elderly because it feels like their needs are often met at the expense of the nation.
Medicare, for example, represents a huge amount of US entitlement spending [2] - though this is somewhat of a misleading number because Medicare funds quite a few things besides strictly healthcare expenses (like it funds most physician residencies, IIRC). Coming out in opposition to Medicare is often viewed as a bit of a political death sentence, because your primary goal as a politician is (at least it feels this way sometimes) to pander to older voters.
Additionally, steadfast conservatives tend to be elderly [3], which often puts the elderly in opposition to younger generations. Since politicians tend to listen primarily to their actual voters, you can sort-of understand the young liberal's decree that "things will be better when all the old people die."
Now, none of this is really the elderly's fault (aside from maybe that last part). Medicare is a reaction to an out-of-control healthcare system, and that's not necessarily your grandma's fault. If politicians listen to the people who vote for them, the logical answer would seem to be "go out and vote", not "get angry at the people who do vote."
But, if you're someone who does vote but feels like your voice still isn't being heard, you can probably start to understand some of the animosity.
1: http://www.post-gazette.com/news/nation/2011/05/04/U-S-votin...
2: https://www.cms.gov/research-statistics-data-and-systems/sta...
3: http://www.pewresearch.org/fact-tank/2014/07/09/the-politics...