I've never seen a reason given by a Bible-believer to explain why people suddenly started living far shorter lives.
I've seen several such explanations, typically along the lines of "their DNA was more nearly perfect before the Flud" or "the Vapor Canopy blocked ultraviolet light and there was more oxygen in the antediluvian atmosphere." Pure BS and adhoccery, of course, and YEC's never actually say "antediluvian" - too many syllables.
Well, we live longer, on the average, because of food additives like preservatives, fertilizers, and pesticides. Not too many folks in the industrial world starve any more - largely due to the increased food supply that things like these make possible.
And nobody has ever lived as long as 150 years, except in myth.