Then you can still be able to claim him as a qualifying child. Go back through his information, make sure you did not say he paid for more than his own support.  Also, put he lived with you all year.

 If he files his own return, he must check the box Someone else can claim (his name) on their tax return, on the screen Do Any of These Apply.

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Do I need to file my own taxes if I'm a dependent?

https://ttlc.intuit.com/replies/3302185

The IRS.gov says child under 19. Why does software say under 17?

The age is for 2 different things.  

To claim a dependent they have to be under 19 or under 24 and a full time student.

To get the Child Tax Credit they have to be under 17 on December 31. See pub 972 page 3 Qualifying Child  (it's for 2018)

https://www.irs.gov/pub/irs-pdf/p972.pdf 

what if she made $8900 and is 17? Can I still claim her as a dependent if I supported over 50%

Yes.  You can claim her no matter how much she made if she is under 19 (or under 24 and a full time student).  You just don't get the Child Tax Credit for her if she turned 17 in 2019.

My son earned more than $4,300. Why can't I claim him as a dependent if I supported him the whole year?

Was he a full time student?  Sorry if he is over18 and not a full time student you can't claim anyone if they made 4,300 or more.  

Who can you claim
https://ttlc.intuit.com/community/credits-and-deductions/help/who-can-i-claim-as-my-dependent/01/267...

See IRS Pub 501 starting on page 10 bottom Dependents and Worksheet 2 page 15 for determining support,
https://www.irs.gov/pub/irs-pdf/p501.pdf

WHO CAN I CLAIM AS A DEPENDENT?

You can claim a child, relative, friend, fiance (etc.) as a dependent on your 2021 taxes as long as they meet the following requirements:

Qualifying child

•They are related to you.

•They cannot be claimed as a dependent by someone else.

•They are a U.S. citizen, resident alien, national, or a Canadian or   Mexican resident.

•They are not filing a joint return with their spouse.

•They are under the age of 19 (or 24 for full-time students).

• No age limit for permanently and totally disabled children

• •      They live with you for more than half the year (exceptions apply).

Qualifying relative

•They don't have to be related to you (despite the name).

•They cannot be claimed as a dependent by someone else.

•They are a U.S. citizen, resident alien, national, or a Canadian or Mexican resident.

•They are not filing a joint return with their spouse.

•They lived with you the entire year.

•They made less than $4300  (not counting Social Security)

•You provided more than half of their financial support. More info

When you add someone as a dependent, we'll ask a series of questions to make sure you can claim them.

Related Information:

•Does a dependent have to live with me?

•What does "financially support another person" mean?

•Can I claim a newborn baby?