After you file

Wrong.  If you read the section titled: "Who’s considered a qualifying dependent for the third-round Economic Impact Payment? (updated March ..." in the link that you provided, you will see this:

 

  • Child's age: The child was:
    • under age 19 at the end of the tax year,
    • under age 24 at the end of the tax year, a student, and younger than you, or
    • any age and permanently and totally disabled.