"My Spirit," God is quoted as saying in Genesis 6:3, "shall not abide in man forever, for that he also is flesh; therefore shall his days be a hundred and twenty years." This verse is the source of the Jewish blessing "may s/he live to be 120" (and the delightful Yiddish curse "may s/he live to be 120—without a head"). And despite being pretty old—tracing at least back to the sixth century BCE—Genesis 6:3 is freakishly accurate. The oldest man whose age we can verify died at 116; the oldest woman, 122. What's the deal, and why can't we live longer than that?
ASAP Science explains what we know of the science of aging (which isn't very much) here:
An international team of researchers recently studied the stem cells of Hendrikje van Andel-Schipper (1890-2005) and discovered that, as scientists might expect, 115 years of living had done a number on her telomeres—to the point where almost all of the white blood cells in her body came from the same two stem cells. Does this mean that injecting rejuvenated stem cells into the bloodstream could reduce the effects of aging? Maybe, at least in that particular area. But it's a harder row to hoe when you're talking about tissue, particularly brain tissue. In any case, 120 years seems to be pretty much the high-water mark for natural stem cell preservation.
How the Bible estimated this figure so correctly is a question that can be answered in several different ways, but one good secular explanation is that the Jewish oral tradition recorded the ages of ancestors at death and those committed to remembering it noted that even the longest-lived ancestors never celebrated their 120th birthday. But given how rare supercentenarians would have been in the ancient world, this would suggest a huge sample size recorded over a period of centuries—and may mean that the ancient Jewish oral tradition was exponentially larger than the written tradition that survives today.