Take the age test. Dependents over the age of 18 must be full-time students under the age of 24 in order for you to claim them as a dependent, unless the dependent is permanently and totally disabled. In the case of a disabled dependent, no age limit exists. The dependent claimed must also be younger than you and, if filing a joint return, your spouse.
Ensure that the adult dependent will not claim himself on his personal tax return. An individual can be claimed on only one tax return, so if the dependent is claiming himself, you cannot claim him too.
Enter the dependent's information in the Exemptions section of Form 1040. The information required includes the dependent's first and last name, Social Security number, and relationship to the tax filer.
Include the over-18 dependent in the total number of dependents claimed in the Exemptions section of the Form 1040.
Complete the remainder of your tax return and file the return as instructed.