Thinking a bit about this and David Coleman's position, I think the issue can be thought about in faux-formal terms, even if we don't have enough knowledge to predict the future - specially about the "soft" political and cultural parameters.
Being a bit silly: A population P with an age structure A and an economic system T (technology -including medical technology relevant to the productive phase of the lifecycle- plus organizational methods plus laws) will be able to honor a variety of societal pledges at a level L(P, A, T).
The projected equilibrium situation in the mid- to long-term doesn't look well right now. Alternatives:
- You lower L: Pension reform, an smaller state, less public investment in health and education, etc. Basically, you are producing less, so you do less. Of course, if you cut costs in health and basic science while having the most inefficient military acquisition system on the face of Earth then this is no solution at all, but properly applied it is, at least, a solution. And it's clearly what will happen if nobody does nothing. The problem: this probably isn't what the people who voted the governments in had in mind, so expect some, ah, friction. And reduced living standards for everybody who wasn't already rich in 1999.
- You raise P: Immigration! On the face of it, it's the perfect solution. After all, Europe, the US and Japan already import a lot of what they need. Why not people? The problem: there isn't a single modern country who isn't failing at the task of attracting, educating and integrating migrants at a significant scale, save perhaps for the US. (The US is failing big time at the problem of educating, integrating and not screwing over their low and middle classes, but that's another issue... although perhaps it's the same; geographical origin is less relevant than integration in the global network, after all is said and done.)
- You "fix" A: "Everybody start making babies now!" Sounds fun, but it's just not doable. People can be "convinced" of having less kids -if you happen to be a totalitarian enough country- but having more kids is a whole different matter. Children are expensive, bothersome (I know, I was one), and cramp your style. It'll take a lot to give enough incentives for people in a modern economy to have more kids, and that's the problem: governments already have (or think they have) more pressing hundred-billion-a-month problems.
- You raise T: aka the sci-fi solution. That's my favorite solution, and I think it's also David Coleman's. A longer productive life thanks to advanced medicine, nootropics and more exotic gadgets. Massive retooling of your IT infrastructure, followed by more massive retooling, and so on ad infinitum or at least until Intel gives up. Flexible labor rules, a population that has given up on ever retiring, and productivity growth trend high enough to both compete with the rest of the world and compensate demographic trends. The problem: getting the political and cultural support for such a radical revolution. A lot of people are squeamish about stem cell research (here is where I roll my eyes; who the hell feels proprietary about their stems?) and are declaring war against memory enhancing pills or genetic treatments. The IT revolution was comparatively easy, but faster computers won't be enough to compensate for less and older workers. You'll need to upgrade your workers... Cue a lot of editorials about the spiritual dignity of progressive cognitive decay, lowering libido and general organ failure as you approach a three digits age.
Roughly speaking, it looks to me like the US is going the L- and T-ways, although only despite their government's best efforts to keep them in the 1950's, and it'll eventually have to stop being scared of Al-Qaeda to focus on the really important thing (i.e., this). Europe has toyed with A, but it's not working, and neither is P, but I give them points for trying. L is creeping in there, and it'll break a lot of hearts (and politicians' careers). Japan is trying T (at least when it comes to robotics), but for some reason they aren't going at it properly (as an anime fan, I have to say that the failure of Japanese business to live up to the obvious, glorious, glaring blueprints in their popular culture is disappointing).
And at some time, while everybody is dealing with this, oil prices will keep going up (even when nothing is being blown anywhere releveant) , coasts shift with an slow and devious pace, agricultural yields change all over the place, etc, etc.
But I try to cynically contemplate one thing at a time.