There are two types of dependents, "Qualifying Children"(QC) and standard ("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.
So yes, being over 19 and less than a full time student, means she cannot be a QC and must meet the Qualifying Relative rules to be your dependent for 2020.
A person can still be a Qualifying relative dependent, if not a Qualifying Child, if he meets the 6 tests for claiming a dependent:
- Closely Related OR live with the taxpayer ALL year. Note it does not have to be both as the other answer said.
- His/her gross taxable income for the year must be less than $4200 ($4300 in 2020)
- The taxpayer must have provided more than 1/2 his support. Note how this test is slightly different from the support test for a QC
In either case:
- He must be a US citizen or resident of the US, Canada or Mexico
- He must not file a joint return with his spouse or be claiming a dependent of his own
- He must not be the qualifying child of another taxpayer
