Methuselah was the oldest man to ever live. When something is called "as old as Methuselah," it means that it is really old. Gen. 5:27 Thus all of the days of Methuselah were 969 years, and he died.
Job 15:7 “Are you the first man who was born? Or were you brought forth before the hills?" Job is about a man questioning God about why he is being punished. One of Job's friends accuses him of thinking that he is more important than he is.
Job 19:20 My bones stick to my skin and to my flesh, and I have escaped by the skin of my teeth. Job is talking about how he is barely living. In popular culture, to escape by the skin of your teeth is to barely survive.