This is somewhat like the premise to a book called Gone all the people over 14 vanish and once you turn 14 you can choose to leave but no-one knows what happens. The books are extremely twisted with just crazy shit going down. (Kid kills himself because he is super depressed by driving a car off a cliff and the kids he lived with retrieves cooks and eats him.) Yeah that kind of weird. Also kids have weird powers and shit. I love this series to death though.
This is somewhat like the premise to a book called Gone all the people over 14 vanish and once you turn 14 you can choose to leave but no-one knows what happens. The books are extremely twisted with just crazy shit going down. (Kid kills himself because he is super depressed by driving a car off a cliff and the kids he lived with retrieves cooks and eats him.) Yeah that kind of weird. Also kids have weird powers and shit. I love this series to death though.
Also probably not this but sounds pretty similar.
"Gone" by Michael Grant.
https://www.amazon.com/gp/aw/d/0061448788/ref=tmm_pap_title_0?ie=UTF8&qid=&sr=
Fits: people disappear except kids, magic/monsters, first book in a series.
Missing: cover art.
Unsure: I haven't read it yet, so I'm not sure if the boy is a god or not.