Am I considered a dependent if my parents pay for my college, but im 21

If you are a full time student and don’t provide more than half of your support and you live with them when not in school they can claim you. 

It depends, there are requirements to be claimed as a dependent on someone's tax return. In your original question, you said your parents pay for college but you also mention that your grandparents pay for your college. But either way, here are the qualifications for being able to claim you as a dependent:

Qualifying child

• 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).
• They didn't provide more than half of their own support for the year.

Qualifying relative

• They don't have to be related to you (despite the name).
• 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 lived with you the entire year (exceptions apply).
• They made less than $4,400 in 2022.
• You provided more than half of their financial support.

Even if you qualify as someone else's dependent, they have the option to claim or not to claim you. 

But since you had a job, I would recommend taking a look at your w-2 Box 2 and see if federal income tax was withheld. If they do in fact choose to claim you as a dependent but had federal income tax withheld from your paychecks, you can still file a tax return to possibly get your federal income tax refunded back to you. Just be sure to select "claimed as dependent on someone else's tax return" when filing your taxes.