Shem was 100 year old when his son Arpachshad was born two years after the flood. That would have made him 98 when the flood came, and thus the second born of Noah. (Genesis 5.32 tells us that Noah was 500 years old when he had sons; Genesis 9.22-24 tells us that Ham was the youngest.) So, Japheth was Noah’s oldest son.

Shem is often mentioned first because Abraham was a descendant of Shem, and Abraham was the father of God’s people.