A person does not need income to file a return and claim the rebates.
However, if a person qualifies to be claimed as a dependent, they must answer "yes, I can be claimed as a dependent" even if the person who could claim them, doesn't want to claim them. Answering "yes" will disqualify them from the rebate. If they falsely answer "no, I can't be claimed as a dependent" to get the rebate, that may be considered tax fraud, although no one knows how much effort the IRS will put into enforcing the issue. The IRS has 3 years to audit all returns and 6 years in the case of fraud.
The round 3 payment does not have an age limit on dependents, if you claim your child as a dependent for 2020, you will get the round 3 payment for them.
Your 19 year old child who is not a full time student can be claimed by you as a dependent if,
a. you paid more than half their support
b. they have less than $4300 of taxable income.
