Hmmm… sounds like a SF story I was planning to write… one of these days… (Except in my scenario life extension is prohibitively expensive)
First, population - I think it was Freakonomics, in their discussion of birth rates and abortion, pointed out that thanks to birth control, women have the number of children they want, typically - if they accidentally start early, then they don’t have children later. If we posit a medical breakthrough where moderately good healthy children are possible from 15 to 350, for both men and women - first, the birth rate drops since there is not the pressure to “hurry up, your clock is ticking”. Many people may put off settling down and having kids until they hit their 100’s, or 200’s. Get the house paid for, travel all 7 continents, spend a month in a Tibetan monastery and a year or decade working with unfortunates in the third world. So the rate at which the population increases will not be the same as our current one since childbearing will be put off for decades or centuries. Yes, some may find it enticing to raise a new set of family members every few decades, but I suspect as many will be happy not being tied down.
That’s another scenario not mentioned - yes, the first world populations will have mainly long-lived people, but the third world will be a collective of immortal elite lording it over a drudge workforce of natural life mortals. Their incentive would not be to raise the economic level of their country, as they would lose the benefit of cheap servants, plus the risk that diluting the financial pool by paying the lower classes more means they would be priced out of the rejuvenation market too.
Which brings up the equivalent first-world question. What happens to the Walmart greeters and restaurant staff of the first world? There is always a class of person that makes themselves poor. The ultimate socialist medicine would be to make this treatment available to all. If it costs the same as say, buying a new car every 5 years over 20 years - who wants to help pay that for the 10% (20%? 30%?) who cannot afford it? Certainly, the problem people - those with mental problems or addictions, who cannot make the payments, will die slowly while their peers are a buff 40 years old for centuries. (So - is there a cut-off? Once you are biologically, say, 60 years old it’s too late for the treatments to save you? Gives new meaning to a “life sentence” if the prison system does not pay for these treatments) Watch for underground quacks doing back-alley half-assed rejuvenation treatments…
Depends too on the cost. If all your surplus earning are gobbled up by your 20-year treatment round, it puts a whole new spin on financial planning. Yes, once the governments and pension planners realize that there are people who can collect benefits for centuries, a whole different regime will kick in. they will pay you when your treatments begin to fail, rather than 65. Or, not at all - you’ve had 350 years to plan for this. What happens to investment? At a certain point, the system reverts to steady state, but for the 300-plus year adjustment, things will be different. There’s already a suggestion that Japan and its stagnation may be the poster child for the coming global economy. A surge of baby boomers created the modern world, and took the best part of it and socked it away into retirement funds, chasing elusive returns from bubble to bubble (such as the recent mortgage bubble). Now, they boomers with the big retirement funds are actually retiring and cashing out. Will some investment assets become a glut on the market as everyone liquidates at the same time? Will savings actually diminish in value?
Of course, all that rejuvenation money goes somewhere. Who gets rich off that? Rejuvenation medical specialist will be a big new in-demand career.
What happens politically? Older voters are typically more conservative. But then, these voters will be most focussed on what policies mean for their access to rejuvenation. Higher taxes? no thanks. Education? There’s a workforce from the last 100 years who already know how to read and write, already have job skills, ensuring the up and coming workers are ready is less of a priority. Maternity leave and benefits will disappear. Why would we pay someone to bring more people into an overcrowded world? “If you wanted to have kids, you had 100 years to plan and save for them.” Health care? will we look at people who have some conditions the way we look today at whether it makes sense to treat someone already incurable and dying so they can live a few more months? If they’re too sick to afford to rejuvenate in the next 20 years, why spend money on someone who will be gone in 20 to 30 years? More focus on curability of diseases and condition, not treatments to prolong mere survival.
As for jobs - at the lower level will be the people scrabbling to pay for the treatments every 20 years. They will live those lives of “quiet desperation”. No time for a relationship, or kids that subtract from their ability to save. Will they go postal when they reach the deadline and realize they can’t afford the treatment? If I’m going to die in 30 years, I’ll go out and take as many of these lucky bastards as I can." These people will work for the same dead-end job or perhaps whatever they can find for 300 years. the well-off middle class will probably do a job for a few decades, then switch careers through boredom or lack of challenge. “I’ll never be a top-notch musician after 40 years, so let’s try airline pilot or investment banking.”
One theory is people will be more careful - but the other theory is that some will translate the boredom into risk-taking; rock climbing, skydiving, high-speed racing along mountain roads, running with the bulls, etc.