In order to be a qualifying child for dependent purposes, your child must be under age 19 or, if a full-time student, under age 24. There's no age limit if your child is permanently and totally disabled.
Once they turn 24, they can only be claimed as a "qualifying relative", and must meet the gross income test ($4,700 in 2023). If they meet all of the tests (live with you all year, are supported by you, and didn't file a joint return with a spouse if they owed tax), they can be an "Other Dependent" which is a smaller credit ($500).
See this TurboTax tips article for more information.
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