Points to Note
When Lamech died,
Shem would have received at best a second-hand account of the Creation story / Fall of man from either Lamech or Methuselah (as mentioned previously) OR third-hand account from Noah.
When Noah was 600 years old (the flood water came):
This means Methuselah died in the year of the flood!
The name "Methuselah" means "His death shall bring". Methuselah was God's stopwatch for the wicked. Whilst Methuselah was still alive, the wicked still had time to repent, but once he died, that time ran out - and the flood came.
God was nevertheless patient with the wicked since Methuselah was the longest person alive in the Bible. God gave them all 969 years in which to repent.
God laid to rest all the righteous men first before the flood (except Noah). He did not want to destroy the righteous with the wicked together.
When Noah died,
Shem was 448 years old
Arphaxad was 348 years old
Shelah was 313 years old
Eber was 283 years old
Peleg already dead for 10 years
Reu was 219 years old
Serug was 187 years old
Nahor already dead for 9 years
Terah was 128 years old
Abraham was 58 years old (Isaac not born yet)
Abraham would have received at best:
When Abraham died,
So, like Abraham - both Isaac & Jacob would have received at best: