"Why should we assume Cain was the first born of Adam when Adam was 130 yrs old when Seth was born?"
Because this is what is recorded in Genesis, Chapter 5.
5 This is the book of the generations of Adam. In the day when God created man, He made him in the likeness of God. 2 He created them male and female, and He blessed them and named them Man in the day when they were created.
3 When Adam had lived one hundred and thirty years, he became the father of a son in his own likeness, according to his image, and named him Seth. 4 Then the days of Adam after he became the father of Seth were eight hundred years, and he had other sons and daughters. 5 So all the days that Adam lived were nine hundred and thirty years, and he died.
6 Seth lived one hundred and five years, and became the father of Enosh. 7 Then Seth lived eight hundred and seven years after he became the father of Enosh, and he had other sons and daughters. 8 So all the days of Seth were nine hundred and twelve years, and he died.
Ancient Jewish records indicate that Adam and Eve had 56 children, 33 males and 23 females.
Yes, these 56 offspring of Adam and Eve, brothers and sisters, bore children in what we would call incest today. Abraham married his half sister, and she gave birth to their son Isaac.
Why did Adam have his first child at age 130 years? The answer is that childhood lasted far longer than it does today. Even as late as 100 years ago, pre puberty for females was 14 years of age and 16 years for males. Today, females and males reach puberty reach to 2 to 3 years earlier due our changes in our diet, including eating meats from animals being fed growth harmones.
Studies in neanderthals (bones) reveal that they reached puberty in their 30's. Scientists derived from examining their bones, only certain bones grow during childhood, and other bones grow in adult hood, and that alone make them look different to Modern homosapiens.
Incest was not banned by God until the Mosaic Laws was established on Mount Sinai about 450 BC ( located in Arabia near Midian).
Why did God ban incest in 450 BC?
My answer, IMO, is that when God created Adam and Eve, their DNA was perfect, and so the DNA of their immediate childrens DNA was perfect and very near perfect in geneartions to come. However, God also applied the 2nd Law of Thermodynamics to our universe, by which all physical things deteriorate over long periods of time, including human DNA via random mutations to the DNA.
Incest causes the inbreeding of deteriotating mutations to our DNA, and thereby fixing Diseases in our genome due to these DNA copy errors in the inbreeding populations - including such diseases as Hemophila, Tay-Sachs, Cystic Fibrosis, Sickle Cell Disease, and many more.
For this reason i believe God banned incest in the Mosaic Laws, to avoid these diseases due to harmful copy errors becoming fixed in their DNA of family lines.
According to Physics, our whole universe is dying in accordance with the 2nd Law of Thermodynamics. They call it a heat death when work no longer can be done.