A recent report explained this issue well:

QUOTE: ...The key to curbing any excess activity by the immune system apparently rests with Carabin, a newly discovered protein made by the specialized white blood cells that march in when a virus attacks...

When people are infected with a cold virus, for example, the virus enters cells and hijacks its works so that the cells become viral factories. The immune system's white blood cells go after these infected cells not only by fielding chemicals that kill them directly, but also by turning on genes that help out. When Liu and his group added Carabin to cells and then studied such genes, they discovered that Carabin disabled the "on" switches, keeping the genes off...

Tracking Carabin to its origins, the researchers said they were surprised to learn that viral infection not only turns on the immune system machinery, but also triggers the making of Carabin, which in turn shuts off the immune response.

"It's like having a built-in timer to keep the immune system in check," says Liu.

If Carabin turns out, after further study, to be a keystone natural inhibitor of immune responses, Liu added, it may prove useful in stopping such unwanted immune reactions as the rejection of transplanted organs...  UNQUOTE.

Source of the quoted material: http://www.sciencedaily.com/releases/2007/01/070117134429.htm

Here are some factors that would need to be taken into account when assessing why people supposedly live longer today:

1. The percentage of women who died during childbirth then (whenever that is) as opposed to now.

2. Deaths due to diseases that are rare today, but that are usually easy to prevent or cure, such as TB.

3. The living conditions, for example, people not having refrigerators, yet living in cities like New York, where it can get very hot in the summer.

4. The fact that jobs were more hazardous 100 years ago.  Compare the percentage of the population of the US that were coal miners then as compared to now, for example.

5. The fact that many substances were not known to be dangerous, and thus were no avoided, as is usually the case today.

6. The much higher death rates among infants and children.

7.  In some time periods, the number of deaths of young men due to war.

8. Various epidemic deaths, which again is rare today, even if death due to "HIV/AIDS" is taken into account in nations like the USA.

9. Vitamin or mineral deficiencies, which are not nearly as common today.

10.  Lack of medical knowledge, which would mean that many died of things that could be easily prevented today.

11.  Lack of medical technology or equipment.

12. Lack of today's drugs, some of which are useful, of course, while others keep people alive longer than would otherwise be the case.

Thus, any claim that today's diet is better because people appear to live longer is quite presumptious, and would require a degree of evidence analysis that is not possible (due to the inability to quantify these factors as well as a lack of evidence for the "Industrial Revolution" period or previous epochs).