I am not sure whether the secret for longevity lies in the genes or in the lifestyle. But you can't say that humans aren't designed to live that long. There are references from 400 or 500 year ago, about people living up to 100 year or more. As the living standards increase, along with the advancement in the medical science, longevity will increase further.
It's been repeatedly proven to be possible, but that doesn't mean it's viable or intended. Mother nature has little use for a 115 yr old quietly farting in the corner of the cave.