If your 20-year-old full time student daughter did not provide for more than half of her own support, you can claim her as a dependent, however large her income may be. Time away for school is counted as time living at home.
Please revisit her info entered in TurboTax, especially the questions on the points mentioned above.
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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.
The $4200 income limit, you may have heard about only applies to standard dependents (Qualifying relatives)