pleasing aroma

This refers to the good smell of the roasted meat.

said in his heart

Here the word "heart" refers to God's thoughts and emotions.

"do very serious harm to the earth"

This can be made more explicit: "because mankind is sinful." (See: rc://en/ta/man/translate/figs-explicit)

"from their earliest years they tend to do evil things" or "when they are young, they want to do evil things"

Here the word "hearts" refers to people's thoughts, emotions, desires, and will. Alternate translation: "their tendency" or "their habit"

This refers to an older child. Alternate translation: "from their youth"