You may claim your 21 year old daughter as an exemption on your taxes provided that the followin additonal requirements are met:
1. she is a student enrolled as a full time student in an accredited college (as your qualifiing child; or as your qualifying relative provided the following are also true.
2. She earns less than 3300 dollars a year
3. She lived as a member of your household for more than half the year
4. You provided for more than half of her support.