Then you would have two retirees for every three working age people. And given that only ~70% of working age Americans are actually working, it would mean three non-working adults for every two adults with jobs. Either the retirees would have a really low standard of living, or workers would get much smaller share of the value they create than they currently do.

If you are not already retired or close to retirement, you should assume that the normal retirement age is ~70 years. Anything lower cannot be sustained with the current demographic structure.