Here are the rules for 2023 from pub 596 on age of qualifying child.

Age Test Your child must be: 1. Under age 19 at the end of 2023 and younger than you (or your spouse, if filing jointly); 2. Under age 24 at the end of 2023, a student, and younger than you (or your spouse, if filing jointly); or 3. Permanently and totally disabled at any time during 2023, regardless of age.

If you have a qualifying child there are 7 basic rules for everyone, and age is not one of them. 

If you are sure the IRS denied your client only because of age then you need to correct the IRS by calling the number on the letter she received.