The 65 year old of today is far healthier and fit than the 65 year old of 1935. Simple fact.
Actually, it's far more subjective. Yes, the average 65 year old is more fit today, but there isn't
that much difference in the health of an average 65 year old today than in 1935. The main reason the average lifespan has increased is because medical development has provided far more effective treatments for the maladies that effect older individuals. Cancer, heart attacks, and strokes are substantially more survivable today than they were back then. Longer lifespans are not due to the frailty of old age setting in later. It's because frailty is more survivable.
But more to the point, I think you missed the underlying problem. Increases in average fitness longevity do not match increases in average life longevity in a year-for-year manner. Currently, the average lifespan for American males is 77.5 years. Back in 1935 it was 60 years. That's a 17+ year increase. If we envision that increase continuing the same rate, then 80 from now we could expect an average lifespan of 95 years old. Do you really think it's reasonable to expect people to continue working until they are 87 years old (your proposed retirement age of 70 plus 17 years to account for increased life expectancy)?
Your suggestion to index the retirement age to population demographics is interesting. But I think it would be too volatile to be effective. It would be subject to yearly variations in birth and death rates. At the current moment, the oldest 9% of the population is older than it will likely be when Generation X starts to retire. The retirement age would constantly be going up and down. There would be instances of people reaching their retirement age one year but dropping below it the following year.