On Bereishis 5:32, Rashi brings an explanation that Noach had children when he was 500
so that Japheth, his eldest son, should not be liable for punishment before the Flood, as it is written (Isa. 65:20):“For the youth who is one hundred years old shall die.”
But what about all the other people under 100 at the time the Flood came? Why did they die, if they were under the age of culpability?