Local demographics will vary wildly but it looks like the baseline would be 16% 65 and older, 13% in the range of 55-64. Since we're talking about prisons we should probably ignore the population under 18, so the normalized expected population over 65 is closer to 20%.
That's federal prison data. People who go to federal prison aren't necessarily the same cross-section of the population who go to state prison.
Anyway, those numbers are certainly much younger than the median age in the USA nationally, which is 39 years with over 65s being 15% of the population.