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.
Your son cannot be a QC because he is too old. But he can still be a Qualifying Relative, if his income was less than $4400. TurboTax is doing it right.
