Adam and Eve were not necessarily the first humans from point of view of biology. It is a metaphor the meaning of which is hard to figure out. It says in the Talmud "the works of creation are not taught to two" meaning the true meaning is only taught by a Rabbi to one student in private.

We must understand that God created the earth and everything in it, finally he created Adam and Eve to manage all this that he had created giving them the opportunity to live in the garden of Eden where they had everything, for disobedience they are expelled so they were the first humans outside the garden of Eden and all the others were their offspring.

Well, the Bible says exactly that Seth was born when Adam was 130 years old. Many believe that Adam was born at 30... but, well, Adam's lifespan is counted from zero in adulthood, so it's a relevant and irrelevant detail at the same time, hahaha.

What I was getting at is, assuming Cain was the firstborn, and since Adam and Eve had no fertility problems, perfect genetics, no issues with feminism, etc., hahaha, he could have already had more than 30 children by the time Cain killed Abel, and an estimated population of grandchildren and great-grandchildren of over 160 people or more. So, without a doubt, we don't know at what age Cain left or at what age he married... but it was definitely a relative, hahaha.