A mandatory retirement age would have to be between a certain age rather than an exact age. If we just have a single set age, what if people don't even live to reach that age? They spend their whole lives working and not even able to use that retirment they worked so hard to earn.
If some simple research about life expectancy is done, the results produced will indicate that the average life expectancy of an American has increased. This is in spite of poor health practices. The basic reason for this is scientific advancement. The elderly right now are draining Social Security to the point of bankruptcy. When the system was designed, life expectancy was much lower and only minor adjustments have been made to account for this. If the elderly are still working, at least they still pay into the system a little bit. Requiring them to retire will negate that all together.
While the nuances of this particular issue may differ from other similar issues, the fact remains that there should be no reason to withhold work from someone purely on the basis of old age. While some people may be unfit to work by 65, others may be perfectly able and willing to work into their 80s. I see no reason to punish some people in the hopes of protecting others.
While I can understand the theory of pushing older workers out to make room for younger workers, this would be a blatant case of age discrimination. Additionally, I find it hard to believe that all US citizens would be able to save up enough to retire at the same age. What then? How can the government be expected to support those citizens when Social Security is already running at a deficit?
No, a mandatory retirement age would not benefit society, because people who want to keep their minds sharp by working should be allowed to. Also, some people might have had financial crisis early in life, and they might need to continue to working to support their family. Continuing to work in old age would also likely cut down on senility.