We do not know when Cain and Abel were born. If you apply the curse of 100 years from Isaiah 65, which is the reality of life with death and decay limited, you get the Math.
Adam was 30 years in the Garden of Eden. God gave him Seth after waiting the 100 years mentioned as being cursed in Isaiah 65. Jesus was also 30 years old when He was baptised. The common sense approach is that Adam was created with an age of 30.
The Catholics came up with them being children and innocent, following the pagan lead of Satan's fable of PANdora's box. Pan is Satan. The Pied Piper, Peter Pan and the children taken to never land. All this deception of loosing children to adulthood, or the attempt to save them by philosophical means. Sell your soul to the devil and loose your children in the process.
We do know from Genesis 2 that the 7th day was a Lord's Day. Peter says do not be ignorant about a Lord's Day. It is 1000 years. God rested from a perfect creation without death, and let it multiply for 1000 years. Adam did not age during that time. Then God created the Garden on the "8th" day. It was 1000 years later for the sons of God plural. Adam was one of many created in God's image both male and female on the 6th day. Adam was not even named until after Adam named the animals. When God placed Adam in Paradise, He gave him a name and Adam was the spiritual leader preventing sin from entering creation. That lasted 30 years. Cain and Abel were born in the Garden before sin during those 30 years. Adam had to wait 100 years after being banned, before God sent Seth. Seth had to wait 105 years. The time span went down until Methuselah. Then it seems that Methuselah had daughters first, before any sons. The daughters the sons of God decided to marry, thus starting the corruption of the sons of God.
The millennial 1000 years will be similiar and after 500 years and dozens of generations, the nations of the millennium will start getting restless and 400 years later will be ripe for Satan to once again deceive humans even under the leadership of Christ. The Flood destroyed the earth the first time. At the GWT, fire will destroy the earth.