Can I claim my child as a dependant after 18

You never put a child's W-2 on your own tax return.   Your child can prepare and file her own tax return in order to seek a refund if federal or state tax was withheld from her paychecks.

If she is still 18 at the end of 2024, she can be claimed as your dependent for 2024.   Here is the criteria for claiming a dependent:

IRS interview to help determine who can be claimed:

https://www.irs.gov/help/ita/who-can-i-claim-as-a-dependent

WHO CAN I CLAIM AS A DEPENDENT?

You can claim a child, relative, friend, or fiancé (etc.) as a dependent on your 2023 taxes as long as they meet the following requirements:

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 $4,700 in 2023.   (For 2024 —  $5050)
• You provided more than half of their financial support.

When you add someone as a dependent, we'll ask a series of questions to make sure you can claim them. There may be other tax benefits you can get when you claim a dependent.

MY DEPENDENT HAD A JOB

If your dependent has a W-2 for his after-school job, summer job, etc. you do not include the information on your own return. You can still claim your child as a dependent on your own return.  He/she can file his own return for a refund of some of his withheld wages (he won’t get back anything for Social Security or Medicare), but MUST indicate on it that he can be claimed as a dependent on someone else’s return.  (Supervise this closely or prepare it for him!)

If your dependent’s earnings were over $400 and were reported on a 1099Misc or 1099NEC then he must file a return and pay self-employment tax for Social Security and Medicare.

You might also want to use free software from the IRS Free File versions:

https://apps.irs.gov/app/freeFile/

If she is under the age of 19, lives in your home for over 6 months during the year and does Not provide for over one-half of her own support then you should be able to claim her as your dependent under the Qualifying Child rules.  Her income is not relevant for claiming her as a dependent under the QC rules.

You do Not enter her W-2 on your tax return,  If she had taxes withheld from her income then she should file a tax return to get a refund of the taxes withheld.  If she does file a return make sure that she indicates on her tax return that she can be claimed as a dependent.

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 can claim her as a qualifying child if all these tests are met
• she has the same principal abode as you for more than ½ the tax year. Temporary absences like for school are ignored
• if she is not a full-time student, she’s under 19 at the end of the tax year. If a full-time student, she’s under 24 at the end of the tax year.
• she hasn't provided over ½ his/her own support
• she didn't file a joint return unless there was no tax liability but merely filing jointly to facilitate refund of taxes withheld or estimates paid

she'll need to file her own 1040 if she wants to get back any federal withholding (box 2 of w2), 

if there's a state income tax then filing requirements are usually different.

with desktop both yours and her return can be done and efiled

with online she'll need her own a/c using a unique user ID but you can use your email address. 

with online -