> a large proportion of people could work in some job up to age 80 (possibly after changing jobs)
It would be an interesting question given most of the aged people change their job. Then those categories of jobs would probably be overrun by aged people seeking those jobs, and hence poor wages and rampant unemployment.
It doesn't have to be the same job, or to be gap-free, though. If you have a strong retraining/welfare system to cover the gaps, as Denmark has, it's not as big a deal if someone needs to take a year or two off to change jobs. If you can't do your job anymore at some point for health reasons, there is a generous system for disability leave. And, a common unemployment insurance system (a-kasse) covers being unemployed for up to two years, with up to ~$36k/yr salary and retraining / job-finding assistance. As a last resort, there's a floor of ~$22k/yr welfare that anyone can get if they have no other source of income, with no time limits. That's comparable to the low end of U.S. Social Security, so at least nobody would really end up with no income.
The wager is that enough people would be productive post-65 to have a significant overall effect.