Is this a qualifying child or qualifying relative? How old? Student?
If your child is 24 they are no longer a qualifying child. In order to be able to claim them as a dependent, they would need to meet the rules for a Qualifying Relative. In order to be a qualifying relative, one of the qualifications is that their gross income is less than $4,050. For a qualifying relative, if their income is greater than that, then they will not qualify regardless of whether or not you are providing most of their support. <a rel="nofollow" target="_blank" href="https://www.irs.gov/publications/p501#en_US_2017_publink1000220954">https://www.irs.gov/publications/p501#en_US_2017_publink1000220954</a>
Thank you! I kept thinking 25 was the threshold - my mistake.