I have the same question and situation which was 90% answered by reading the above. There is one caveat I have though. Just like the original poster, my son was 18 on January 1st 2024 and graduated High School in June 24, while working after school and summer. He has already filed his own returns without help from me back in January. The only thing I do not know is if he checked that box stating "Someone else can claim me as a dependent".
To my understanding, as long as he did check that box I can still claim him. The unfortunate part is that he has since left for the Navy and I cannot contact him to ask if that box was checked or not (likely he would not remember either).
What would happen to my return if I still claimed him as a dependent but he did NOT check that box?
