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.
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