There are two types of dependents, "Qualifying Children"(QC) and Other ("Qualifying Relative" in IRS parlance even though they don't have to actually be related). There is no income limit for a QC but there is an age limit, student status, a relationship test and residence test. Only a QC qualifies a taxpayer for the Earned Income Credit. They are interrelated but the rules are different for each.
The personal info section, for dependents, will ask the questions to qualify your dependent. When it sees that your child is over 18, it will ask for her student status.
If she is over 23, she cannot be a QC, even if she is a full time student.
