Retirement? Nah, never heard of it.

They shouldn’t be allowed to pick the groceries if they’re not staying for dinner.

For real. I'm in my 40s and I vote for the world my kids want.

Unlike you though, boomers are selfish as hell. They don't care if they drop dead tomorrow, they want to force their world on us from beyond the grave.
- boomers are selfish as hell
  I don't like this kind of statement.
  It's far too general to be either true or false. Lots of zoomers and millenials are too.
- Most republican voters I know, boomer or otherwise, simply view voting differently than most their lefter-leaning constituents. I often hear them say that the point of voting is to simply choose what benefits you the most, and that if everyone simply chooses things that specifically align with their own wants and needs, that the biggest, most important groups will get what is needed. It's not even that they understand that they're being selfish by only voting in their own best interests, they just honestly believe that considering the needs of others when voting undermines its effectiveness.
  Now, it's obvious that they're wrong - smaller groups deserve just as much of a say as their larger counterparts, and the country benefits when they do - but they don't think about it that way. I believe it's also why republicans are so concerned about becoming a minority - they honestly believe that voting should specifically only benefit the largest group, and are desperately trying to maintain "their people" as the largest group.
My wife and I talk a lot about how we don’t know what causes are coming down the pipeline in the next decades, so our only goal is to have ability to go “I don’t know what this is, I don’t know what the answer is, and nobody has to earn my agreement to keep me from obstructing it.”
If we raise our kids right they’ll do the right thing. My buy in isn’t that important beyond how much they want my participation.

I wonder if senators genuinely believe they’re doing good by hanging in there. Someone’s gonna sit them down and have a tough talk. You know? Be like “hon, you ain’t helping. If you want to help, pick up litter or plant some native species. Your political goofing is hurting the rest of us.”

As someone not from the USA, looking at things as an external observer, it's really weird for me to see this increasing age-related hate. Instead of focusing on removing fascists, conservatives and the ultrarich from power, you're focusing on age as if it's the determinant factor on someone's character and intentions, and even promoting more age-related stigma. I even wonder if it's some sort of coordinated mass manipulation to prevent the increasingly radicalizing people from taking out the groups in power, by letting them fight against the older ones, while they prepare younger fascists to run the place, but that would be too much of a conspirational thought. Anyway, good luck with that, let's see how all this will develop.

I know that the issue is not age, but look, I've been waiting my whole life to see these people retire and leave jobs for my generation, and leave political office too. so for me, even though i know its not some singular solution to something, i still would just like to see the old people retire, as i have been waiting all my life for them to do so.
It's not the age itself at all, look at Bernie Sanders, still sharp and still popular. This comic seems pretty much about Diane Feinstein, and the knock isn't exactly their age, it's related to their age. They're grossly out of touch with regular Americans any more and we think the age is a factor. We want fresh blood, new eyes, more forward thinking than some of these career politician dinosaurs seem capable of anymore.
Every year a new batch of Ivy League Valedictorians and Rhode Scholars emerge and we're stuck with old folks running the show.
I even wonder if it's some sort of coordinated mass manipulation to prevent the increasingly radicalizing people from taking out the groups in power, by letting them fight against the older ones, while they prepare younger fascists to run the place, but that would be too much of a conspirational thought.
It's not much of a conspirational thought; it's common sense blanket chaff laying.
Ageism, racism, and ableism are easy and timeless levers to pull. Who did the Nazis blame for their problems (besides Jews, their own built-in trifecta of isms)?
Edit: Nazis are just topical; pick any other axis power. Pick any civilization on Earth whilst they were getting their subjugating in, for that matter. I speak of human nature "always has been.jpg" monkey reaction shit.
Othering works best when it's programmed to work at a glance, and as demographics shift, age becomes as useful as racism to manipulate if you're being paid by a handful of shit stains coordinating their self-defined supremacy (Billionaires. I mean the oligarchical dragons who own the information outlets).
Too much of a conspirational thought? Please. It's not even worth a line item in Repub Think Tanks strategy meeting; it'd be like saying the sky is blue or money makes you worthy for that kind. Axiomatic.

I wonder what it is about the US system that leads to retirement-age presidents, while in most countries I know about that's not a thing at all

Probably related to the split one-party system.
Money. Older people tend to have money

Technically, former GOP Speaker of the House Paul Ryan retired at like 48 after royally fucking up the deficit and raising retirement age for everyone else.

RIP, RGB... but not too peacefully, I hope.

RGB
Ruth Gader Binsburg I assume?

Another possible final caption:
"Well, YOU start running for office, then."

Like the old fucks would vote for us instead of other old fucks