JOEL - Lehi left Jerusalem about 600 BC(1 Ne 2). Jacob was born at approximately 590 BC sometime in the wilderness after Lehi left Jerusalem(1 Ne 18:7). At about year 421 BC Enos said(remarking that he himself was old) it has been 179 years since Lehi left Jerusalem(600-421=179)(Enos 1:25), which would make it about 169 years since Jacob was born(590-421=169). If Jacob fathered a son(Enos) at 79 years old, and Enos lived to be 90 years old then 79+90=169. The only unusual thing about this is that Jacob would have fathered a son at a fairly old age.
Abraham fathered a son at the age of 100 (Genesis 21:5) and lived to the age of 175 (Genesis 25:7), while his son Isaac died at the age of 180 (Genesis 35:28) and his grandson Jacob lived to be 147 years old.
Another thing to consider is that the Book of Mormon, like the Old Testament, makes no use of “grand” relationships. A grandson of Jacob may have also been named Enos who actually wrote the book of Enos, but he would be refered to as a son of Jacob rather than grandson, and the second generation Enos would have refered to his grandfather as simply father.