After that Cain said to his brother Abel: “Let us go over into the field.” So while they were in the field, Cain assaulted his brother Abel and killed him.- Genesis 4:8
The Bible account does not give us his exact age at the time of his murder, but his approximate age can be deduced. Since the Bible records nothing in the way of history between the expulsion of Adam and Eve from the garden of Eden and their producing of their first two sons, Cain and Abel, it is reasonable to conclude that the boys were born within the space of a few years after their parents’ fall into sin. But as for the next-recorded events, leading to the death of Abel, it is written that these did not take place until after “the expiration of some time.” (Gen. 4:3) How much time?
Well, the third recorded son of Adam and his wife Eve was born shortly after Abel’s death and when Adam was 130 years of age. (Gen. 5:3) At the time of Seth’s birth Eve said: “God has appointed another seed in place of Abel, because Cain killed him.” (Gen. 4:25) It is not at all likely that Eve would have said this if a great many years had intervened; she evidently desired a son to replace the murdered Abel, and so the birth of Seth must have occurred shortly after Abel’s death. It is possible, then, that Abel may have been as much as a hundred years old at the time of his martyrdom.