In Matt 24:34 Jesus said the generation that sees the signs He was describing would not pass away until they have all been fulfilled. The average life span is 70-80, but a man could also by great luck live to be 100. You don’t think Jesus meant that even if there was one 100 year old man left at His second coming that would count, do you?
I think that would be an extreme interpretation of Matt. 24:34 and would not qualify the verse as the sign the Lord intended it to be. How would anyone be able to determine who the last living person was? Remember, the disciples asked Him for signs of His coming and of the end of the age. Signs have to be easily understood to achieve their purpose.