In order to receive the credit the child has to be under 17 at the end of the year.
For the second stimulus money, you could receive it for a child that is 17 as long as they turned 17 in the second half of 2020.
@Hal_Al I found the information again in the following article:
The IRS will use people's 2019 tax returns to determine their stimulus payments, which means that teens who hit their 17th birthday in the second half of 2020 — after tax returns were due to the IRS — could still qualify.
Also, my granddaughter turned 17 in Nov. of 2020, my son did get the $600 for my granddaughter who turned 17 in Nov. 2020.
Thanks. I read that to say: you may have still gotten a check in 2020 for a child who turned 17 in 2020 (a loop hole). But I don't take it to mean you can file a 2020 return and claim the rebate credit.