Being we only have the information of his three sons, I think we have to go with that, but Noah may have had daughters before his 3 sons. They naturally got married, moved away, and were not part of his family anymore, or made it into the ark.
Your question brings up the conflict between the Biblical account, the customs in the Middle East, and the traditional Christmas story in Western Christianity. Kenneth E. Bailey addressed this question in the chapter, "The Story of Jesus’ Birth: Luke 2:1–20," in his book,
Jesus through Middle Eastern Eyes: Cultural Studies in the Gospels. IVP ...
The same question should be asked in a different way:
Why did not Joseph return to Hebron?
Why did not Joseph return to Ur of the Chaldees from whence Abraham came?
Why did not Joseph return to Jerusalem which was known as the "City of David"?
There is a simple explanation for all this which is integral to the story. Note ...
How is Joab related to Amasa?
The topic "Amasa" in the Insight on the Scriptures lists Amasa as Joab's cousin:
1. Son of David’s sister or half sister Abigail and Jether (Ithra), and cousin of Absalom and Joab. (2Sa 17:25; 1Ch 2:16, 17) Jether is called an Israelite in Samuel and an Ishmaelite in Chronicles, perhaps because he lived in Ishmaelite ...
@Ozzie Ozzie has it correct! If you do the math and assume healthy births for all three sons...
3 Children × 9 Months = 27 Months
27 Months ÷ 12 Months = 2.25 Years/2 Years 3 Months
2 Children × 9 Months = 18 Months
18 Months ÷ 12 Months = 1.5 Years/1 Year 6 Months
This means Japheth would've been 9 Months at Shem's birth, and 1 Year 6 Months old at Ham's ...