Child turning 18 this year

A child that turns 18 in the current year will not impact whether they are a dependent or not, but they are not eligible for the Child Tax Credit.  With that being said, however, they could qualify for the Credit for Other Dependents, which is a credit up to $500 (note the current Child Tax Credit is up to $2,000 per child).

The tests to be your Qualifying Child are as follows:

1. Relationship Test: The child must be your son, daughter, stepchild, foster child, brother, sister, half brother, half sister, stepbrother, or stepsister, or a descendant of any of them.
2. Age Test: The child must be (a) under age 19 at the end of the year and younger than you (or your spouse if filing jointly); (b) under age 24 at the end of the year, a student, and younger than you (or your spouse if filing jointly); or (c) any age if permanently and totally disabled.
3. Residency Test: The child must have lived with you for more than half of the year.
4. Support Test: The child must not have provided more than half of the child’s own support for the year.
5. Joint Return Test: The child must not be filing a joint return for the year (unless that joint return is filed only to claim a refund of withheld income tax or estimated tax paid).

So, the fact that your child is turning 18 this year does not affect the dependent status.  However, fast forward to 2025 - the year that your child turns 19, so by definition they will not be "UNDER the age of 19".  Of course, they still may be considered your Qualifying Child, provided for example, that they are a full time college student.  if for 2025 they no longer meet the Qualifying Child tests, then you will want to look to see if they meet the Qualifying Relative test. If they meet neither, then they will not be your dependent for 2025.

Additionally, since your child is 18 this year, they will not qualify you for the Child Tax Credit.  However, they should not have qualified you for the Child Tax Credit on your 2023 tax return - as to qualify for the Child Tax Credit, they had to be "Under age 17" at the end of the year. If your child is turning 18 in 2024, that means they would have been 17 at the end of 2023, and therefore not eligible for the Child Tax Credit.

Keep in mind that there is also the Credit for Other Dependents - so if your child is age 17 or older at the end of the year, but meets the rules to be your dependent, you can qualify for the $500 Credit for Other Dependents.