How old was Cain when he killed Abel?

[tonychanyt]

The Bible did not specify Cain's age when he killed Abel. I can only guess.

Adam and Eve were expelled from the Garden of Eden in Ge 3.

Ge 4:

1 Now Adam knew Eve his wife, and she conceived and bore Cain, saying, “I have gotten a man with the help of the Lord.”

Assume that Cain was their firstborn.
Assume that Eve had lived 2 years when she gave birth to Cain.

3 So in the course of time, Cain brought some of the fruit of the soil as an offering to the LORD.

The Bible did not say specifically how long was this course of time.

8 Cain spoke to Abel his brother. And when they were in the field, Cain rose up against his brother Abel and killed him.

25 Adam knew his wife again, and she bore a son and called his name Seth, for she said, “God has appointed for me another offspring instead of Abel, for Cain killed him.”

Sounded like Seth was born soon after Cain killed Abel. Let's say a year after.

Ge 5:

3 When Adam had lived 130 years, he fathered a son in his own likeness, after his image, and named him Seth.

Adam had lived 130 years when Seth was born.
Adam had lived 129 years when Abel was killed.

Cain could be 127 years old when he killed Abel. That's the upper bound of my guess.