Hello, this is my first post here. This answer is quite easy if you believe that the authorised version (king james version, kjv) is the only inspired word of God. Many false bibles are sent into the world to decieve many..
so, here`s the answer.
If you look at Gen 5:32 -
And Noah was five hundred years old: and Noah begat Shem, Ham, and Japheth.
Noah was 500 when he got his three sons. After that you will see in Genesis 6:3 -
And the LORD said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years.
NOTE: nowhere does it say here that people will live up to 120 years. God said that human race (except the people in the ark) will be wiped off in 120 years. Note also that gen 5 and gen 6:1-10 are not chronologically ordered. God said this before Noah got his kids. In Gen 6:10 there`s the proof.
This means that God said he will wipe out human race when Noah was 480. In the meantime Shem, Ham and Japeth were born, and it took him a hundred years to build the ark. 480+120=600, Noahs age when the flood started.
NOWHERE does the Bible say that people won`t live more then 120 years.
Hope this helped.