There are two types of dependents, "Qualifying Children"(QC) and standard ("Qualifying Relative" [QR] in IRS parlance even though they don't have to actually be related).
There is an age limit (24) for being a QC, but not for a QR. There are plenty of people over 23 who can still be a dependent.
See full rules at: https://turbotax.intuit.com/tax-tools/tax-tips/Family/Rules-for-Claiming-a-Dependent-on-Your-Tax-Ret...
