How do you know that Ham was the youngest son?

Answer:

"So Noah awoke from his wine, and knew what his younger son had done to him" (Genesis 9:24).

The word "younger," or "youngest," depending on your translation, comes from the Hebrew word qatan. It is an adjective which can be translated as "small," "insignificant," or "young." Typically, it is used to compare two people's ages and identify which one is younger. When used in a comparison of more than two items, it functions as a superlative to identify the youngest (or smallest, or least significant) of all being compared.

Since there are three sons, the word qatan means Ham is the youngest of the three. A similar example is found in I Samuel 16:11 and 17:14, where David is identified as the youngest of Jesse's sons.

This causes problems because it is typically understood that children are listed in birth order, yet the three sons are always listed as "Shem, Ham, and Japheth." Noah was 500 years old when he had Shem, Ham, and Japheth (Genesis 5:32). It is likely listed this way because Noah had his sons in quick succession. Noah was 600 years old when the Flood began (Genesis 7:6). He was 601 when the Flood ended (Genesis 8:13). Two years after the flood, when Shem was 100 years old, he had his first son (Genesis 11:10). This would mean that Shem was born when Noah was 503. Since Ham is the youngest, this leaves Japheth as the eldest if Noah had his first child at the age of 500.

What is also interesting is that the Bible states that Japheth was the eldest brother. "And children were born also to Shem, the father of all the children of Eber, the brother of Japheth the elder" (Genesis 10:21). Unfortunately, the phrase is ambiguous in Hebrew and could be translated as "Shem, the eldest brother of Japheth." You'll find translations split fairly evenly as to which way it should be translated. But given Shem's age when his first son was born, it is more likely that "the brother of Japheth the elder" is the correct translation.

In both Genesis 10:1-2 and I Chronicles 1:4-5, after listing the three sons, it is Japheth's descendants who are first broken down in the following generations; another indication that he is the eldest. But in those lists, Ham's descendants are listed second and Shem's third.

Therefore, we must conclude that while the Bible tends to list children in birth order, it is not a hard-and-fast rule that can always be counted upon.