John Gill 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 when the flood of waters
was upon the earth
When it began, for he was in his six hundred and first year when it ended (Genesis 8:13), his eldest son was now a hundred years old, since when Noah was five hundred years old he begot children (Genesis 5:32).