If you burn now it still hurts. When he asked if you would like to live forever he didn't mean like wolverine where you heal in seconds.. Just extending life for a few thousand years.
Curious, how did Noah live for 500 years? I think somewhere in the bible this is mentioned, OP I'm sure you know.
I don't know how Noah lived for 900+ years, 300 of which were after the flood. But, some thoughts...
1. Many people of the time before the flood lived hundreds of years. Maybe all who didn't die at the hands of other people, or through some natural accident, lived several hundred years, at least.
2. The atmosphere of earth was different before the flood. There was more moisture up there. This moisture may have blocked solar/cosmic radiation which, after the flood, got through, and gradually changed the DNA in people so they gradually lived shorter lives.
3. We see a tremendously large number of plants and animals in the fossil record that are no longer existing on earth. There may have been nutrients from them, the plants especially, that extended life. Such would no longer be around due to species extinction.
4. Some people think that the sun changed drastically in some way, so that it spews out more and different harmful radiation, while in the past it radiated beneficial energies that are gone today. Google "sungazing."
5. Some people think that we have forgotten traditional exercises that people normally did years ago, exercises that extend life for almost all people who practice them daily. Along with this would be concious practice of positive attitude.
6. Google "pyramid power."
None of these things (and others) may be enough alone. We might have to "have" these things and others to live a long life.
Note that religious people who dedicate themselves to their religion, often live longer lives - https://bitcointalksearch.org/topic/health-and-religion-1373864.