Yes, you can claim your 18 year old child as a dependent under the Qualifying Child rules if they meet all the requirements.
To be a Qualifying Child -
1. The child must be your son, daughter, stepchild, foster child, brother, sister, half brother, half sister, stepbrother, stepsister, or a descendant of any of them.
2. The child must be (a) under age 19 at the end of the year, (b) under age 24 at the end of the year and a full-time student or (c) any age and permanently and totally disabled.
3. The child must have lived with you for more than half of the year. Temporary absences while away at college are considered living with you.
4. The child must not have provided more than half of his or her own support for the year.
5. If the child meets the rules to be a qualifying child of more than one person, you must be the person entitled to claim the child as a qualifying child.
6. The child must be a U.S. citizen or U.S., Canada or Mexico resident for some portion of the year.
7. The child must be younger than you unless disabled.
You may be able to claim your child as a dependent but those are not the tests.
To claim a child as a "qualifying child" dependent, they must meet the following tests:
1. Live in your home more than half the year, except for temporary absences. Being away at college or a boarding school is considered a temporary absence.
2. The child does not earn more than half their own financial support. It does not matter who provides the other half, as long as the child provides less than half their own support.
3. The child is your biological child, or step child, or legally adopted child, or your grandchild, or niece or nephew, or younger sibling.
4. The child is under age 19 on 12/31/2020, or is under 24 and is a full time student.
5. The child does not file a joint tax return with a spouse.
There is another set of rules called "qualifying relative" dependent, and the tests are,
1. The child has less than $4250 of taxable income and does not file a tax return, or only files a tax return to claim a refund of withholding and claims no other deductions or credits.
2. You pay more than half the child's financial support.
For your own child, it doesn't matter where they live for the qualifying relative test.
All these rules are built in to the Turbotax dependent interview, if you answer the questions, you should get the right answer.
Note 1: there are special rules for children of divorced or separated parents, although they may not apply once the child turns 18. Ask for more details.
Note 2: for a child in college, it is sometimes difficult to determine if the child provides "more than half" of their own support. The child's total living costs include food, clothing, travel, entertainment, medical care and insurance, housing, and tuition. Support that the child provides includes their income, or withdrawals from savings that they spend on themselves, as well as student loans in their name. Support that the parents provide the child include the value of free housing while the child lives with them, as well as money spent on clothing, travel, medical care and insurance, and tuition and room and board. Grants and scholarships don't count on either side of the equation. Sometimes, if the child has large loans in their own name, they end up providing more than half their own support when everything is added up.
In other words, if the child lived with you for at least 6 months and one day and did not provide 1/2 of his own support, you will be able to claim him as your Qualifying Child. Whether or not you provided insurance or he was in school is irrelevant. Next year, when he turns 19, he will need to be in school to be claimed as a qualifying child.
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