Thank you for your question. 

For the purposes of the eligible dependant credit, the dependant may be your parent or grandparent, or a child under the age of 18 who is your child, grandchild, brother/sister through birth, adoption, marriage or common-law partnership. Thus, you may not be able to claim the eligible dependant tax credit for a 21-year-old.

 However, in some cases, you may be able to claim this credit for a dependent who is not a minor.

To see your eligibility to claim this tax credit please follow the Canada Revenue Agency link - Answer these 2 questions. 

Here is a TurboTax link that you may find helpful: Claiming the Amount for an Eligible Dependant Credit.

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