The child tax credit has not changed. You can get the CTC for a child who is UNDER the age of 17. You lose the CTC in the tax year that the child turns 17.
One item has changed, slightly. If you have less earned income in 2020, than you did in 2019, and that results in less of the "Additional" child tax credit and Earned Income credit , you may elect to calculate your 2020 ACTC & EIC based on your 2019 income. This in the new Covid tax rules