Lex Luthor's age isn't fixed in the comics. It can change based on the creative direction of the writers and the era of the story. Generally, he's portrayed as an adult in his prime, often in his late 30s to 50s.
Well, Lex Luthor's death varies depending on the comic storyline. Sometimes it's due to his own schemes backfiring, other times it's at the hands of a powerful foe.
In self insert as Lex Luthor fanfiction, the theme of intelligence and innovation is quite prevalent. Lex is a brilliant mind, and the self - inserted character often gets involved in his scientific and technological endeavors. This can range from helping him create new inventions to competing with him in the realm of intellect. Additionally, themes of isolation and loneliness might be explored as Lex is often portrayed as a rather solitary figure. The self - insert can either exacerbate this feeling or provide some sort of companionship to him.