When God first appears to Abraham – which, by the way, was what the big guy renamed Abe – his name was originally Abram – Abram is 75 years old, although that doesn’t mean much, since his father Terah has not long died of old age. Terah lived to be 205, so no doubt Abe anticipates a number of good years ahead of him yet.
God tells him a bunch of stuff – that he should move from where he lives (in what is now Iraq) to Canaan (or what is now Israel); that he will become the founding father of a great nation; that he should change his name; and that his wife, Sarai (also renamed as Sarah) will soon become pregnant. Sarah is old enough to be unable to bear children, so she laughs at this prophecy, although one assumes that it seems less funny after she conceives and delivers Isaac, as prophesied.
As mentioned in:
Earth Song — Michael Jackson