Provide the same services with about 60% less revenue, and NOT by inflating the difference away. Oh and without hiring essentially anyone.
Keep raising the retirement age so someone born after ~1980 essentially never retires. And ... I guess pray that medicine actually makes this possible.
Both have obvious problems. It seems to me a near-certainty that all governments will not know how to run fast enough to option b. Now there may be "robots" or whatever as a plan, long term, but while I don't work in robotics, I do have a master in it. We're 50 years away from viable robotic care for humans. We're currently not capable to build a robot that's physically capable of doing it without weighing 800+ kg, which makes it a non-starter (and this is the only part of the problem I expect someone like Elon Musk might be able to fix in, say, 10 years)
Of course, governments being what they are they are, are working on it. By:
1) making immigration much harder (and world demographics will start making it harder very soon now, and those won't stop)
2) dropping investment in robotics and medical research
3) making it harder every year to become medical care professionals
TLDR: you definitely won't retire in 20 years. Unless we can create a company and make, at the barest minimum, ~5 million there won't be retirement for any of us.