From AARP.org -> There are reasons why the CDC is recommending this priority order. So far, in late spring and early summer, we saw that the worst cases [of H1N1 flu] were not in people over the age of 55. The people who’ve been the sickest are those who are totally naïve to the virus. Older people are lucky enough to have some immunity to it.”
In tests run by the CDC, a third of blood samples from people over age 60 contained antibodies capable of fighting H1N1. An article published online July 13 in Nature reported that the very old—those who lived through the devastating 1918 influenza pandemic—had high levels of the antibodies in their blood.