There is absolutely no reason to believe that Noah was already older than 120 or that he lived longer than a normal life span. There is every reason to believe that nearly everyone's body falls apart between 50 and 120 if disease or malice hasn't gotten them prior.
When you fabricate its better if the fabrications concern things either long ago or far away and if the present situation is different from that described you have to explain why if you don't want your audience comparing present reality to prior and seeing through the hoax.
Example if you wanted to posit that insert legendary figure here was 30 feet tall and came from a village where most were 20' and up you can't set the story in the next village last year because someone has probably been there and noticed that people just aren't that tall.
If it was a thousand years prior across an ocean you can claim that people became less mighty as the years went by and lost touch with their gods which plays to people's tendency to filter the past through nostalgia.
If a 30 foot tall giant seems less likely than a 900 year old man then you may need to apply critical thinking skills.
Just because its still socially OK to believe nonsense doesn't mean its true.