John Calvin Commentary Genesis 7:6

"And Noah was six hundred years old when the flood of waters was upon the earth." — Genesis 7:6 (ASV)

And Noah was six hundred years old. It is not without reason that Moses again mentions Noah's age. For old age has this among other ills: it tends to make people more indolent and morose. Consequently, Noah's faith was all the more conspicuous because it did not fail him in that advanced stage of life.

And just as it was a great virtue not to languish through successive centuries, so his promptness likewise deserves great commendation, because when commanded to enter the ark, he immediately obeyed.

When Moses shortly afterwards adds that Noah had entered on account of the waters of the deluge, these words should not be interpreted as if he were compelled by the rushing of the waters to flee into the ark. Rather, moved by fear because of God's word, he perceived by faith the approach of that deluge which all others ridiculed. Therefore, his faith is again praised here because, indeed, he raised his eyes above heaven and earth.