How were people able to live for hundreds of years before the Flood?

Ages of the patriarchs

Prior to Noah’s Flood, people were recorded to live for over 900 years.  Methuselah lived 969 years (Gen. 5:27). How could they live for so long?  There are different theories, one being the Canopy Theory.  Another theory is that the oxygen levels at the beginning of time were different.  Today, the atmosphere has 20.9 percent oxygen in it.  What if in the past, the oxygen percent in the air was higher, so people breathed in healthier air, free of the harmful smog and pollution we have today.  This would allow people, plants, and animals to live longer, and this would explain why we find some very large fossils of animals that today are much smaller.  Still another valid reason is that during the time before the flood, the rate of harmful mutations present in the DNA was much lower.  People had overall much better health and were able to fight off diseases.  Today there are more diseases and even mutations of diseases, causing many health problems for all of us.  It is now rare to live past 100 years.