The is is reply to your 2/12 post was the mother of all living? (fivedoves.com)

As you know, the Bible doesn't give Adam & Eve's age at the time that Cain & Abel were born.  It only tells us that Adam was age 130 when Seth was born according to Gen.5:3.  However, if you read the same account of Cain's birth in the Book of Jubilees, Jubilees 3:34 says, "And they had no son till the first jubilee, and after this he knew her."  Jubilees 4:1 says, "And in the third week in the second jubilee [64-70 A.M.] she gave birth to Cain, and in the fourth [71-77 A.M.] she gave birth to Abel, and in the fifth [78-84 A.M.] she gave birth to her daughter Âwân."  So both Adam & Eve were between 64-70 years old when Cain was born.  They were between 71-77 years old when Abel was born, and between 78-84 years old when their daughter Awan was born.  

And what about their descendants?  Some of Adam & Eve's male descendants were older than 70 years of age when their first son was born.  According to Gen. 5:6, Seth was 105 when he begat Enos.  Enos was 90 when he begat Cainan (Gen. 5:9). Cainan was 70 years old when he begat Mahalaleel (Gen. 5:12). Mahalaleel was 65 years old when he begat Jared (Gen. 5:15). Jared was 162 years old when he begat Enoch (Gen. 5:18). Enoch was 65 years old when he begat Methuselah (Gen. 5:21). Methuselah was 187 years old when he begat Lamech (Gen. 5:25).  Lamech was 182 years old when he begat Noah, and Noah was 500 years old when he begat Shem, Ham & Japheth (Gen. 5:32). 

Shem was 100 years old when he begat Arphaxad 2 years after the flood (Gen. 11:10), but it wasn't until Arphaxad that the male descendants had children at a much younger age than 65 or 70 except for Terah.  Arphaxad was 35 years old when he begat Salah (Gen. 11:12).  Salah was 30 years old when he begat Eber (Gen. 11:14).  Eber was 34 years old when he begat Peleg (Gen. 11:16).  Peleg was 30 years old when he begat Reu (Gen. 11:18). Reu was 32 years old when he begat Serug (Gen. 11:20).  Serug was 30 years old when he begat Nahor (Gen. 11:22). Nahor was 29 years old when he begat Terah (Gen. 11:24). Terah was 70 years old and begat Abram, Nahor & Haran (Gen. 11:26).  Why did these men have sons at a younger age than their male ancestors?

So who's to say why Adam & Eve didn't have any children until they were between 64 to 70 years of age and why did the first 10 Patriarchs from Adam down through Noah live longer lives than the descendants that came after them?  Look at Abram he was 86 years old when he had his first son Ishmael by Hagar (Gen.16:16), and he was 100 years old (Gen.21:5) when his wife Sarah, who was 90 years old had their 1st son Isaac. (Gen. 17:17), but Sarah had been barren.  We won't know the answer to your questions until we get to heaven as to why couples didn't have children in their 20's.