One major reason members of the Wizarding world live longer than normal humans is very simple: they have much better medicine than we do. We see Madam Pomfrey completely regrow Harry's arm from nothing, at one point, and she's a school nurse. I imagine the things that commonly lead to death of old age in humans (cancers, heart disease, neurological conditions) really pose no difficulty to magical medicine, especially at someplace like St. Mungo's.

As an aside, this can be seen in our world, too. If you look at the life expectancy of the Dark Ages (around mid 30's on average), we have made much progress towards a Wizarding lifespan. The medical community has been postulating for the past decade that if we could overcome a few of the more troubling diseases we commonly face, then we could live lives well over a hundred years long.

One major reason members of the Wizarding world live longer than normal humans is very simple: they have much better medicine than we do. We see Madam Pomfrey completely regrow Harry's arm bones from nothing, at one point, and she's a school nurse. I imagine the things that commonly lead to death of old age in humans (cancers, heart disease, neurological conditions) really pose no difficulty to magical medicine, especially at someplace like St. Mungo's.

As an aside, this can be seen in our world, too. If you look at the life expectancy of the Dark Ages (around mid 30's on average), we have made much progress towards a Wizarding lifespan. The medical community has been postulating for the past decade that if we could overcome a few of the more troubling diseases we commonly face, then we could live lives well over a hundred years long.