The Bible indicates that Noah had three sons prior to the Flood (Genesis 6:10). Noah’s sons were not all the same age, but let’s first begin with Genesis 5:32:
After Noah was 500 years old; he became the father of Shem, Ham and Japheth.
This indicates that Noah was 500 or just over when his first son was born. Listings such as these are rarely an indication of ages but shows that Noah began having children when He was 500 (Genesis 11:26). The listing of children often started with most important one (through Shem we receive the blessing of Christ). We know the relative ages of each from Genesis 10:21 and Genesis 9:24.
Sons were also born to Shem, whose older brother was Japheth; Shem was the ancestor of all the sons of Eber.
When Noah awoke from his wine and found out what his youngest son (Ham) had done to him.
Genesis 10:21 indicates that Japheth was the oldest and was born when Noah was 500 years old. Ham is the youngest, as indicated in Genesis 9:24, after Ham’s inappropriate actions to his father.
Therefore, Shem had to be born in between Japheth and Ham. Shem wasn’t born as a triplet or twin of Japheth when Noah was 500, as shown by Genesis 7:6 and Genesis 11:10.
Noah was six hundred years old when the floodwaters came on the earth
This is the account of Shem. Two years after the flood, when Shem was 100 years old, he became the father of Arphaxad.
So, Noah was 600 when the floodwaters came on the earth, and two years later Shem was 100. Therefore, Shem had to be born to Noah when he was 502. We are not sure of Ham’s exact age in Scripture, but he had to be born after Shem. Thus, Genesis 5:32 introduces us to Noah’s sons all together, and other passages give more detail about their birth order and age.