To claim a dependent, you must meet the following five requirements:

Gross Income - To be considered a dependent, the individual cannot have $3,050 or more in gross income. The exceptions to this rule are children under age 19 and full-time students under age 24.

Full-Time Students- A student is someone enrolled in school full-time for some part of each of five months out of the year. If the full-time student is under age 24 and has a job where they earned $3,050 or more, they may still be claimed as a dependent by their parents.
If your child must file a return, they cannot claim their own exemption if the child qualifies as your dependent.
Students are allowed to take a standard deduction for dependents if they are not claiming themselves.

Relationship - A dependent must either live with you for the entire year as a member of your household or the dependent must be related to you.

Citizenship - A dependent must be a United States citizen or resident alien, or a resident of Canada or Mexico, for some part of the tax year in which they are claimed.

Joint Return - Generally, a dependent cannot file a joint tax return with their spouse and still be claimed by you.