One of the challenges of writing speculative fiction (science fiction or fantasy) is that often, your characters are really fucking old and it can be hard to know what to do with that.
The most common solution seems to be to ignore the problem, and treat immortal characters as being whatever age they appear to be. See: every vampire romance story ever, where teenage girls date hundred year old vampires without anyone ever seeing a problem with it in-story (however creeped out some audience members may be). Note that while vampire stories written for teenage girls may be the most conspicuous offenders, the situation does sometimes get gender-flipped. In Buffy the Vampire Slayer, Xander can date a 1100 year old ex-demon without ever being teased about being into older women.
Even when the problem is acknowledged, few authors seem to do much with it (though I’d be interested to hear about exceptions). Cory Doctorow’s excellent sci-fi novel Down and Out in the Magic Kingdom features a narrator with four doctorates who at one point early in the story mentions that, “My girlfriend was 15 percent of my age, and I was old-fashioned enough that it bugged me,” but otherwise his age doesn’t show much. (Googling some of the details just now, I noticed someone describing it as a coming of age story.)
It’s interesting to think that some day, we might get to find out what a bicentenarian or even a millenarian (a person who lives to be a thousand, not an adherent of millenarism) really acts like. Whether we’ll develop the technology to achieve immortality anytime soon is up for debate, but there seems to be no reason in principle why this couldn’t happen eventually, if medical science continues long enough.
And yet… could immortals who don’t act like immortals turn out to be realistic after all? Though I can’t speak from experience, my impression is that many (most?) people don’t change all that much psychologically after age 35 or so. They get settled into a career. Their political and religious opinions are less likely to change while they are young. They often don’t seem to get any better at their jobs.
Even intellectuals and creative types, who you might expect to spend their lives boldly exploring new ideas and new creative frontiers, often seem to ossify. Not totally, but once they have their first great success–their first idea that gets people excited, their first successful novel, whatever–they often stick to variations on a theme, or at least working in the same general vein. (I actually settled on the number 35 for this post in part because that’s when Dawkins published The Selfish Gene; I think it’s fair to say that book has defined his niche since.)
That raises the interesting possibility that a society of immortals wouldn’t just look no older than 35, it could also be full of people who are stuck mentally wherever they were at 35. Imagine the old fundamentalists who will not budge on points of doctrine where even committed evangelicals of the younger generation are changing their views… and then stretch that out over centuries.
One argument I can think of for why this might not happen: the “not changing much after 35 pattern” may be an artifact of the incentives our society gives people, but the incentives might look different if you’re going to be living for centuries. Basically, the way things are right now, once you’ve found your niche it’s probably in your interest to not gamble by making any big changes, but such gambles might be less costly, or even necessary, in a future society.
For example, consider some old fundamentalist, Norman Geisler or Albert Mohler or whoever, trying to decide whether to compromise a bit on inerrancy. It might help them a lot with appealing to the younger generation of evangelicals… but it would only help a little there, too little to be worth the price in alienated existing supporters. But it’s one thing to stick to your guns for a few decades as you grow somewhat out of touch; it’s another thing to become centuries out of touch because you’re not willing to take some chances.
And maybe in a society of immortals, radically shaking up your life every few decades would be as standard as going to high school. Who knows! Obviously a whole bunch of things could affect the shape of a future society, and in some ways I’m offering up relatively conservative scenarios that could be made nonsensical by more radical changes in society (think some of Robin Hanson’s speculations), but it’s fun to think about.