The biggest difference between old and young people is depth of experience. The elderly aren't racist because they never met a person of another race: they're racist because that's how they were taught and trained to act towards other races.
And I think it's more difficult to argue that "undoing previously held beliefs" (old) is as easy as "forming new beliefs where none exist" (young).
Source: Extensive traveling in the rural American south, and I believe psychological findings back up the tendency to cling to preconceptions
So the issue I have with both yours and the GP's position is I feel you fail to take into account that old age wouldn't exist (or at least "shouldn't" if we don't want to breed zombies) so we cannot assume the same phycology will apply with people who are 100 but physically and mentally 20 as currently applies with those who are 100 with failing physiology and cognitive functions.
https://en.wikipedia.org/wiki/Confirmation_bias
https://en.wikipedia.org/wiki/Selective_exposure_theory
https://en.wikipedia.org/wiki/Cognitive_dissonance
These are not things that humans do because they're old (and somehow less plastic / capable than the young). They do them because that's how our brains work once they've acquired information.
If you're going to argue against current neuroscience and psychology findings and beliefs, then you're welcome to cite some sources. "Good mental and physical health" is necessary but not sufficient for creating as many opportunities to have opinions challenged (as younger people).
In the absence of equivalent ability to be challenged and accept new ideas, parent's assertion that a society without death would look very different from our current "each generation overthrows some ideas of the last as they die" seems to stand.
I should stress though that I do feel you're missing my point. I'm not saying people give up attitudes easily. What I'm saying in an infinite life there is infinitely more opportunities for ones preconceptions to be challenged. I'm not saying it's easy for ones preconceptions to be challenged but it does often still happen and thus with an infinite life span there is both significantly more time and significantly more opportunity for this to happen.
What I'm suggesting is that where we don't observe significant adaptation in preconceptions within a shorter time scales, we might discover that humans are more capable to adapt over a much longer scale. We might even beg for change just out of pure boredom (eg I doubt many people could stand doing the same job for hundreds of years). Change pushes us outside of our comfort zone, into new social groups, and most importantly, change can often force us to evaluate our preconceptions through personal experience which is far more valuable to reconditioning than debate and 2nd hand reporting (which is what your articles focused on).
Ultimately though, and as I said to the GP, this is all speculative. However I would be amazed if infinite life + good health didn't affect human psychology given the massive impact it would have culturally and the significance that mortality, the fear of growing old and ill health, all play in human psychology. There are so many variables that would change (economic, social, psychological, etc) beyond just the numbers on our birthday cakes that I find it weird to assume our lives would just be the same but longer.
Two people with difference beliefs experience the same thing and each walk away with their previous beliefs reinforced.
I hear and grant your argument ("What I'm suggesting is that where we don't observe significant adaptation in preconceptions within a shorter time scales, we might discover that humans are more capable to adapt over a much longer scale.").
But you're seemingly only using that to suggest that there will be one impact (towards more flexibility / change). You're ignoring the other side of the sword: what if more experience only deepens our beliefs because of our hardwired cognitive biases? E.g. if its difficult to overcome 20 years of racist enculturation, then what about 200?
The worries I have, and why I bet more on my future than yours, is that the majority of these biases stem from non-conscious processes in our brains. So there's no indication that taking a brain adapted for a 30 year human lifespan and extending it to 200 is going to radically change their physical, biological underpinnings.
However, if we gain the technology to indefinitely extend life, it seems probable that we'd also have the technical capability to modify these logical flaws. So it's possible we'd be able to "debias" brains to a greater degree.
Or, and I'd actually consider this more unlikely and harder, perhaps our understanding of human psychology and neuroscience will have advanced to the point where we can consciously override our own biases in a more reliable and passive manner (e.g. super mindfulness, Dune-esque psychoconditioning).
My main point is that indefinitely extending life and expecting our mammilian, product of evolution brains to just "get it" isn't a bet I'd be willing to take with humankind's culture hanging in the balance.
Ohhh that's a very good point. I hadn't considered that.
You'd probably have a stronger case if you argued about natural selection instead but given the history of humans, it's hard to believe that many would reached the end of their natural lives before getting killed. Certainly not enough and over a long enough period for immortality to have come about.
This is of course all hypothetical. But so is your argument that older people cannot adapt in the same scenario.