I graduated college in 2021. I received a 1098 form. Even though I was not actually in school for a full 5 months, can I claim as a full-time student for 2021?

[KrisD15]

Yes, you only need to be at least half time for at least one academic period. 

If you are a non-dependent student, enter the 1098-T into your TurboTax program and follow the interview for a credit. 

The American Opportunity Tax Credit is based on "at least half-time" and the Lifetime Learning Credit is less than that.

According to the IRS:

"For at least one academic period beginning in 2021 (or the first 3 months of 2022 if the qualified expenses were paid in 2021), the student was enrolled at least half-time in a program leading to a degree, certificate, or other recognized educational credential."

IRS Pub 970

Note-

To be considered as a student in order to be claimed as a dependent by another Taxpayer, such as your parents it needn't be 5 FULL months just any part of at least 5 months. 

[Hal_Al]

To be classified as a full time student, for tax purposes, you only needed to be in school for parts of 5 calendar months. So, for the extreme example, being a FT student Jan 31 to May 1 would qualify.

But, that 5 month requirement is mostly important only if your parents want to claim you as a "Qualifying Child" dependent for 2021 (see rules below) .  As others have said, to claim a tuition tax break, you only needed to be half time, or more, for one academic period (of no specific length). 

There are two types of dependents, "Qualifying Children"(QC) and Other ("Qualifying Relative" in IRS parlance even though they don't have to actually be related). There is no income limit for a QC but there is an age limit, student status, a relationship test and residence test.

A child of a taxpayer can still be a “Qualifying Child” (QC) dependent, regardless of his/her income, if:

1. He is under age 19, or under 24 if a full time student for at least 5 months of the year, or is totally & permanently disabled
2. He did not provide more than 1/2 his own support. Scholarships are excluded from the support calculation
3. He lived with the parent (including temporary absences such as away at school) for more than half the year

So, it doesn't matter how much he earned. What matters is how much he spent on support. Money he put into savings does not count as support he spent on him self.

See full dependent rules at: https://turbotax.intuit.com/tax-tools/tax-tips/Family/Rules-for-Claiming-a-Dependent-on-Your-Tax-Ret...