You do not get the CTC for a child who will turn 18 in tax year 2021. You can only get the child tax credit for a child who will be YOUNGER than 18 at the end of 2021. You will get the $500 credit for other dependents for the older child.
Do not confuse child tax credit and earned income credit. Those are two separate child-related credits. The EIC is not limited to children under the age of 18. If you qualify for earned income credit you can still receive EIC for the college kid.
https://ttlc.intuit.com/questions/1900923-what-is-the-child-tax-credit
https://www.irs.gov/help/ita/does-my-childdependent-qualify-for-the-child-tax-credit-or-the-credit-f...
https://ttlc.intuit.com/questions/1899157-what-are-the-qualifications-for-the-earned-income-credit-e...
https://www.irs.gov/credits-deductions/individuals/earned-income-tax-credit/use-the-eitc-assistant
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