Rashi on TanakhGenesis 5

This is the book of the generations of man. [Meaning:] This is the narrative of the offspring of man. There are many other Aggadaic interpretations.

On the day Elohim created. This tells us that on the same day that he was created he begot children.

One hundred and thirty years. Up to now he had separated from his wife.

Chanoch walked. He was a righteous person but he could easily be induced to turn towards evil ways. Therefore, Hashem quickly took him away and had him die, i.e., before his time. This is why the verse uses a different expression concerning his death, by writing וְאֵינֶנּוּ “and he was not” in the world to complete his years.

For [He] took him. Before his time; as in: “Behold I take from you the desire of your eyes.”

And had a son. From whom the world was built up.

This [one] will bring us rest. “He will give us rest from the anguish of our hands.” Prior to Noach they had no agricultural tools and he prepared [such tools] for them. The earth had been producing thorns and thistles when wheat was sown as a result of Adam horishon’s curse and this ceased in the days of Noach. That is the intent of the word יְנַחֲמֵנוּ. If you do not explain it this way then the meaning of the term is not related to the name [נֹחַ] and you would have to call him Menachem.

Five hundred years old. R. Yudan said: Why is it that all the [previous] generations had children at the age of one hundred years, and this one, [Noach, had children] at the age of five hundred? Hashem said: If they be wicked they will be destroyed by the [flood] waters which will cause grief to this righteous man. If they be righteous, then I would have to burden him to build many arks. [Therefore,] He restrained his source and he had no children until he was five hundred years old in order that Yefes, his oldest son, not be of punishable age before the Flood as it is written: “For the youngster will die when he is one hundred years old.” [Meaning: the age at which he will be] liable for punishment, in future time, and so it was before the Torah was given.

Sheim, Cham and Yafes. Was not Yefes the oldest? But, first you must discuss Sheim who was righteous and who was born circumcised, and from whom Abraham descended ([as is explained] in Bereishis Rabbah.)