Is there a maximum age after which you can't came a dependent child?

To be a Qualifying Child -

1. The child must be your son, daughter, stepchild, foster child, brother, sister, half brother, half sister, stepbrother, stepsister, or a descendant of any of them.
2. The child must be (a) under age 19 at the end of the year, (b) under age 24 at the end of the year and a full-time student or (c) any age and permanently and totally disabled.
3. The child must have lived with you for more than half of the year. Temporary absences while away at college are considered living with you.
4. The child must not have provided more than half of his or her own support for the year.
5. If the child meets the rules to be a qualifying child of more than one person, you must be the person entitled to claim the child as a qualifying child.
6. The child must be a U.S. citizen or U.S., Canada or Mexico resident for some portion of the year.
7. The child must be younger than you unless disabled.

however, if the person can't be claimed as a qualifying child by any taxpayer they may possibly be claimed as a qualifying relative.

thee are the tests

• meets relationship test (same as for qualifying child) or lived in the taxpayers household for the entire calendar year
• their gross income for 2019 is less than $4,200
• you provide over ½ their support

The IRS has an online test where it will tell you if you can claim the person as a deduction.  I found it after asking the question.