I don't know how many people remember the TV show Sliders from the 1990s, but a friend of mine brought up an episode that is sort of germane to the topic of geritocracy. I know I'm dating myself here, but I really liked the show as a teenager, even if I only saw the first ... maybe four seasons. At some point, even good shows can jump the shark, right?
Anyway, it's the Sliders - Third Season episode, The Breeder, which is like a mishmash of Species and Logan's Run. The important point in the episode is that young people are assigned to be forced organ donors for the older and more powerful.
So my friend wants to know what's to stop the elderly from using us for spare parts if they have all the power?
Well...creepy as that may be, I actually think that people over the age of 90 are not really looking for spare parts. At that age, haven't most people made their peace? Also...isn't the surgical survival rate for the extremely old (hey, let's not sugarcoat here), a bit less than it is for the current people in charge (45 to 65-year-olds)?
I imagine we do have some coercive organ donation going on already, probably catering to the current age-group in power, since they are better suited recipients; however, I really don't see that spiking in a geritocracy. In my experience, immortality is not the obsession of centenarians, but rather the young who fear the loss of youth and vigor as much as they fear death. In other words, I think people my age and slightly older are more likely to prey on weaker members of society than the very aged are.
Oh...for a non-aged-related book on this train of thought try Unwind and its sequel by Neal Shusterman. I wish I had remembered that day, but it's been a while since I read it. Talk about an interesting thought experiment!