וַיָּרַח ה' אֶת רֵיחַ הַנִּיחֹחַ וגו' כִּי יֵצֶר לֵב הָאָדָם רַע מִנְעֶרָיו וגו' (ח, כא)
And Hashem smelled the pleasant fragrance... because the nature of the heart of man is bad from his youth... (8:21)
The Yeitzer Hara Not to Eat:
The Ponovizher Rov, Rav Yosef Shlomo Kahaneman zt"l, related that he once went to the home of the Chofetz Chaim zt"l, and found him eating a meal with his young son. The boy was saying that he didn't want to eat anything, but the Chofetz Chaim wouldn't let him leave the table. He kept trying to persuade him to eat.
that the yeitzer of a person is rah from his youth. Rashi explains that as soon as a baby is born, it has a yeitzer hara. Now, let me ask you a question: What type of yeitzer hara could a baby have? What types of bad deeds does it tell the baby to do? The answer is that it tells a young child not to eat so that he will be weak and unable to learn properly!
When he saw the Ponovizher Rov standing by the door, the Chofetz Chaim said, "Do you know why the boy is refusing to eat? It is because he already has a yeitzer hara. The pasuk says
The Chofetz Chaim added, "I have a proof to this. Go look at my non-Jewish neighbor's son. He eats gluttonously. Why? Because the same yeitzer hara that tells my son not to eat tells the gentile to eat a lot so that he will become strong enough to kill Jews."