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.
The support test is different for each type. The support test, for a QC, is only that the child didn't provide more than half his own support. The support test for a Qualifying Relative is that the taxpayer provided more than half the relative's support.
See full dependent rules at: https://turbotax.intuit.com/tax-tools/tax-tips/Family/Rules-for-Claiming-a-Dependent-on-Your-Tax-Ret...
If a person qualifies as your dependent, you do not need their permission to claim them. But, if they claim themselves (or somebody else claims them), you can expect a letter from the IRS and will need to prove you had the right to claim them. They will get the same letter from the IRS and will have to prove they can claim their self.
