The passage in question in Bereshit reads....

      "My Spirit shall not strive with man
       L'OLaM and his days shall be 120 years."

Often , people have used this to suggest that men are not able to live more than 120 years after their birth.  But in reality, this is what people have READ INTO the text.  It is not necessarily what the text is saying, and , in fact, let me propose at least THREE plausible interpretations for this verse;

Among the possible things this could say is...

 Non-personal directive
    1. G-d will bring disaster 120 years after saying this.

 Personal directive
    2. G-d will not let any men live more than 120 years.
    3. G-d will only strive with men for 120 years, but he might live longer.

 The interpretation of #2 I'd rule out because Abraham lived to 175 well after the flood and numerous others are recorded as living past 120.  #1 fits the time line given elsewhere in Genesis/Bereshit.  G-d brought the flood about 120 years after He made this announcement.  #3 also makes sense because l'olam, translated "Forever" in most English translations, might also be translated "to the end of age".  Usually it means end of "time", but could this also mean end of the man's time, rather than the earth's time? In other words,  a man might live 137 years, but the last 17 of those years, G-d quits striving to bring the man to repentance.  Or maybe G-d doesn't start calling until he's 5 or 10 or 13 or something.  It does not say man will only live 120 years, but that G-d would only strive with him for that long.  That doesn't mean man will die after God quits "striving".

So there's a multitude of things this could be saying, but it is NOT necessarily saying that men will not live past 120 years of age.

Another similarly misunderstood verse is Ps/Teh 90:10, where it says..

    "The length of our days is seventy years-
     or eighty, if we have the strength; "
     (Ps/Teh 90:10)

To suggest that this is the normal lifespan of a man.  In reality, Moshe was lamenting that the Israelites were dropping dead at such a young age.  He lived to over 120 himself.
 

Shalom, Joe