The demographic assumptions that were in place with Social Security was started was quite simple:

The age of initial collection was set at 65 years of age, because in the 1930's, that was the age by which 50% of men had died. The original intent of Social Security was to help pay for the maintenance of widows and orphans for those who survived men who died, and the fund depended on the structure that half the men who might have been eligible to collect from Social Security were dead by the time they were eligible, which helped maintain the benefits for the half of men who lived beyond 65.

Today, if we wanted to re-establish that sort of actuarial assumption to maintain the viability of the fund, we'd have to raise the retirement age to the mid-high 70's - ie, by 10 years, to at least 75 years of age.