From the instructions for form 8615:
"Who Must File
Form 8615 must be filed for any child who meets all of the following
conditions.
1. The child had more than $2,600 of unearned income.
2. The child is required to file a tax return.
3. The child either:
a. Was under age 18 at the end of 2024,
b. Was age 18 at the end of 2024 and didn’t have earned
income that was more than half of the child's support, or
c. Was a full-time student at least age 19 and under age 24 at
the end of 2024 and didn’t have earned income that was
more than half of the child's support.
4. At least one of the child's parents was alive at the end of 2024.
5. The child doesn’t file a joint return for 2024."
Note that being a dependent or either just qualifying to be a dependent, does not trigger the need for form 8615 and the "kiddie tax". At ages 19-23, it's the full time student status that triggers the kiddie tax.
