We can assume the age of Cain approximately above thirty, when he committed the Murder. It is well clear that by that time, Adam has many children and they married among themselves at the age of attaining puberty. So by the time, they increased in strength with two or three generations. Cain's fear was either Abel's children or someone else may kill him in revenge.
Please allow some room for imagination without contradicting the bible. There is nothing confusing / contradictory in bible.
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