For you to claim her as a dependent, she would have had to be no more than 24 yrs old on 12/31/2020 and have provided less than 50% of her support for all of 2020 (see other requirements below)
The principal benefit of claiming her as a dependent is that you may qualify for any Education Credits (primarily the American Opportunity Tax Credit) she is eligible for. (There are no longer any "dependent exemption allowances)".
From Who can I claim as my dependent?: In order for you claim your child as a dependent, she would have to meet the following criteria:
- They're related to you.
- They aren't claimed as a dependent by someone else.
- They're a U.S. citizen, resident alien, national, or a Canadian or Mexican resident.
- They aren’t filing a joint return with their spouse.
- They're under the age of 19 (or 24 for full-time students).
- No age limit for permanently and totally disabled children.
- They lived with you for more than half the year (exceptions apply) (full-time students meet this requirement)
- They didn't provide more than half of their own support for the year.
("Support" includes food, lodging, clothing, medical and dental care, education, transportation, utilities, and so forth).
