I might need a better title (It has now been updated), but here goes, anyway:
I've been considering this for a while now. Suppose we reach a point where we can live for centuries, maybe even millenia, then how do we balance? Even assuming we're as efficient as possible, there's a limit for how much resources we can have, meaning an artificial limit at the amount of people that could exist at any given moment even if we explore what we can of the galaxy and use any avaliable resource. There would have to be roughly the same rate of births and deaths in a stable population.
How would this be achieved? Somehow limiting lifespan, or children, assuming it's available to a majority? Or would this lead to a genespliced, technologically augmented and essentially immortal elite that the poor, unaugmented ones would have no chance of measuring up to? I'm sorry if this has already been considered, I'm very uneducated on the topic. If it has, could someone maybe link an analysis of the topic of lifespans and the like?
It is unlikely that society will ever neatly divide into "haves" and "have nots"- be suspicious of sharp divisions. There should be lots of boundary cases, and probably a smooth gradient.
The primary question for answering these sorts of questions, I think, is whether modification to existing agents is more or less effective than modification to new agents. It seems more likely to me that genetic engineering can radically increase human lifespans than interventions in already developed humans- and if that's the case, then there won't really be immortal elites because the younger generation will always have rosier prospects than the older generation (unless a cap is reached, of course). Currently, we would expect a 200 year old programmer to run circles around a 20 year old programmer, because experience is really valuable. But if the 20 year old has 10 generations of intelligence boosts that the 200 year old can't get, then the 20 year old is probably going to win- especially if one of those boosts is easier experience transfer!