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 and student status test, a relationship test and a residence test.
Since you are over 18 and were not a full time student, you can not be claimed as a QC.
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
His/her gross taxable income for the year must be less than $4300 (2020).
The taxpayer must have provided more than 1/2 his support
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