Here is the chart from SSI that
@betarhoalphadelta provided earlier but converted to a spreadsheet:

Then, extrapolating from those figures to guesstimate the figures for people currently 90 and younger:

Let me just say right off the bat that I'm sure the numbers I got here just from extrapolating are NOT exactly accurate and if you have a better source, we can switch but I just wanted something to work with.
When SSI was established in the 1930s a man who made it to 21 had roughly a coin-flip chance of getting to retirement age. As of 1990 (35 years ago) that was up to almost three-quarters. Today it is probably north of four-out-of-five and by the time people my age get there in another 15 years it will probably be close to nine-out-of-ten.
Furthermore, the men who DO make it to 65 are also living a lot longer PAST 65. It went from less than 13 years for the initial SSI retirees to over 15 years for men who retired 35 years ago and is probably around 16 years today and heading for around 18 years by the time my cohort gets there.
This is the biggest reason for the decline in the workers:retirees ratio.
Unless we are going to massively increase taxes and this would necessarily be on middle-class people not just 'the rich' because you couldn't raise this kind of revenue on "the rich" alone, the age is going to have to go up.
Last time they increased it they exempted everyone within about 20 years of retirement. Under today's set-up with a 67 year retirement age that means everyone over 47 (born in 1978) would be exempt. To keep it simple, lets go with 1980 and, following the pattern they used in 1983 we'd be looking at something like this:
- 67 years and 2 months retirement age for persons born in 1980
- 67 years and 4 months retirement age for persons born in 1981
- 67 years and 6 months retirement age for persons born in 1982
- 67 years and 8 months retirement age for persons born in 1983
- 67 years and 10 months retirement age for persons born in 1984
- 68 years retirement age for persons born in 1985
- 68 years and 2 months retirement age for persons born in 1986
- 68 years and 4 months retirement age for persons born in 1987
- 68 years and 6 months retirement age for persons born in 1988
- 68 years and 8 months retirement age for persons born in 1989
- 68 years and 10 months retirement age for persons born in 1990
- 69 years retirement age for persons born in 1991
- 69 years and 2 months retirement age for persons born in 1992
- 69 years and 4 months retirement age for persons born in 1993
- 69 years and 6 months retirement age for persons born in 1994
- 69 years and 8 months retirement age for persons born in 1995
- 69 years and 10 months retirement age for persons born in 1996
- 70 years retirement age for persons born in 1997 and later.
This would dramatically impact the overall system in part because increasing the age works both sides of the equation. It decreases the number of retirees AND increases the number of workers. However, the impact would not begin until 2047 (22 years from now) when the people born in 1980 turned 67 and had to wait an extra two months before they could retire.
Since this system only moves the age two months per year I think you could advance it by a decade to:
- 67 years and 2 months retirement age for persons born in 1970
- 67 years and 4 months retirement age for persons born in 1971
- . . .
- 69 years and 10 months retirement age for persons born in 1986
- 70 years retirement age for persons born in 1987 and later.
By doing that the impact would start a decade sooner, in 2037 which is only 12 years from now when those born in 1970 turned 67 and had to wait an extra two months to retire.