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Apparently, some of the difference in mortality profile is because there hasn't been a major H1N1 flu in quite a while, so older people have some immunity to H1N1 that younger people don't have because their immune systems have never faced such a virus.

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From what I had read (a New Scientist synopsis of a Univ of Wisconsin study, so my facts may be muddled) it was exposure to the 1918 pandemic that provided H1N1 antibody response. Since that flu variant lasted until 1920 we are talking about people who are at a minimum 89 years old. This is not a large enough cohort to appreciably tilt the mortality profile on its own, there is something else at work here as well.

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