My son was 24 at the end of 2017, a student during all of 2017 and I provided more than half of his support. Why does the software not allow him as a dependent because he earned over $4,050?
Your son doesn't qualify for the dependency exemption because of his age and income. To claim your son as a dependent, he must be either your qualifying child or your qualifying relative. Even though your son was a full-time student, he wasn't under age 24 for the duration of 2017 so he isn't considered a qualifying child. He also isn't a qualifying relative because his income exceeds $4,050. For a complete list of requirements for a qualifying child or a qualifying relative, please see this FAQ: Who can I claim as my dependent?