Not for EIP1. The CARES Act, which provides an additional $500 for child dependents, uses the tax code definition of a child—any child you claim as a dependent on your taxes must be under 17 years of age as of the end of 2019. (The Consolidated Appropriations Act, which provides a $600 additional child payment, follows the same rules).
However, if you are not able to claim them as a dependent for 2020 and if they file their own individual tax return, your child could receive both the $1,200 CARES Act stimulus amount and the $600 Consolidated Appropriations Act payment (EIP1 and EIP2, respectively) via the Recovery Rebate Credit.
Note: If your child, age 17 or older, files his or her own taxes, they still will not receive a stimulus payment as long as you are able to claim them as a dependent; their “Dependent of Another” status disqualifies them from receiving payment.
EIP3, from the American Rescue Plan, does qualify dependents age 17 or older for additional stimulus funds. You can read more on our stimulus payments page.