Jamieson, Fausset and Brown on Genesis 5:5

5. all the days ... Adam lived--The most striking feature in this catalogue is the longevity of Adam and his immediate descendants. Ten are enumerated (Ge 5:5-32) in direct succession whose lives far exceed the ordinary limits with which we are familiar--the shortest being three hundred sixty-five, [Ge 5:23] and the longest nine hundred sixty-nine years [Ge 5:27]. It is useless to inquire whether and what secondary causes may have contributed to this protracted longevity--vigorous constitutions, the nature of their diet, the temperature and salubrity of the climate; or, finally--as this list comprises only the true worshippers of God--whether their great age might be owing to the better government of their passions and the quiet, even tenor of their lives. Since we cannot obtain satisfactory evidence on these points, it is wise to resolve the fact into the sovereign will of God. We can, however, trace some of the important uses to which, in the early economy of Providence, it was subservient. It was the chief means of reserving a knowledge of God, of the great truths of religion, as well as the influence of genuine piety. So that, as their knowledge was obtained by tradition, they would be in a condition to preserve it in the greatest purity.