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.Also interesting that Noah and co. all lived to be hundreds of years old, in the book.
There is some evidence that in some relatively primitive cultures that it wasn't completely unheard of to live to very old ages. If you won the genetic, cultural, and geographic lottery and lived somewhere there wasn't a lot of disease-carrying vermin (fleas, mosquitoes, etc), ate a relatively good diet, lived in a time of relative peace, and stayed reasonably active, you probably had a decent shot of living a good, long life.
What really drags ancient mortality down is astronomical infant mortality, dying in childbirth, infanticide, and war. In more advanced civilizations, with lots of contact with other peoples, dense cities, sanitation issues, and so on, you then start worrying about plagues.
So I think ancient peoples could have reasonably come to the conclusion that about 120 was the maximum age and attributed it to God.
Even in recent centuries there have probably been a very few people living to 110-120, the trouble was it was incredibly difficult for us to verify in the absence of solid, verifiable record-keeping.