At the age of 19 or older, a child can only be claimed as a dependent if they attend school "full time". In this case, "full time" means, carrying a full time course load as defined by the school, in at least part of 5 different months of the year.
So, a graduated high school senior, who was a student from January to May, would be considered a full time student for the dependent calculation for that year, even if they attended college only part time in the fall.
However, if he can't be classed as a full time student in at least 5 months of 2018, then he is not a "qualifying child" dependent. And if his income was more than $4150, he can't be a "qualifying relative" dependent. Those are the only two classifications.
