Why do we say that you can live 120 years? We say this because the Holy Bible says so in Genesis 6:3. And the Lord said, “My Spirit shall not strive with man forever, for he is indeed flesh; yet his days shall be one hundred and twenty years.”
120 years is also mentioned in the oldest textbook of internal medicine, the Nei Ching - the 4500 year-old classic of Chinese medicine. Where it says "In ancient times, people lived to be 120 years old with no sign of weakening . . ."
By contrast, the average life expectancy in the U.S. hovers around 78 to 79 years. The reason our life expectancy is so relatively short is because of factors that hasten our biological aging. The good news is that most of these factors are within our control.