Surely it's just a genetic lottery win? Humans certainly aren't designed to live that long and people with perfectly respectable lifestyles drop dead in their 50s and 60s every day.
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.
There are some physiological limits. Approximated limit of average lifespan is about 250 years. If we solve genetic issues then there still would be another limitation due to nature of our brain. To be exact, there is no possibility to remember more than 4000-5000 years of living experience. You won't die but you'll forget anything what happened before last 4000-5000 years of your hypothetically unlimited life.
And there will be no way to recover these memories.
This wouldn't a problem at all if we are able to prolong our life even 10-15 years beyond our genetic limits (which would be a
huge advancement and step forward, still not possible today). The signal patterns that constitute memory can be recorded and duplicated later in a neural implant. Rats have already been successfully tested with such artificial memory brain implants substituting for their long-term memory, thus the technology, however rudimentary as of now, is already here...
Besides, we seem to forget insignificant details of our life and remember the most important (emotional) ones. In short, brain is not a primitive FIFO memory device