Good question. Noah was exactly 600 years old when he entered the ark according to Genesis 7:6. He began having children when he was 500 years old (Genesis 5:32) and Shem, the middle child, turned 100 two years after the flood (Genesis 11:10), so Noah began building the Ark at roughly the age of 500. It took around 100 years to build the ark, which is a very reasonable amount of time for Noah and his three sons, plus perhaps hired labor if they were wealthy in their time.
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