What is the age of taxpayer born 1-1-2000 at 12-31-2024?

Deja vu 

@JWH01 Inside joke referencing a long ago epic thread.

A puce colored gaurd dog.... to continue the inside joke.

Well, we are talking about someone over 16, so at least that part of the issue is taken care of  😀

The IRS has some info, including a link to their interactive wizard. Here:

https://www.irs.gov/credits-deductions/individuals/earned-income-tax-credit-eitc

On a serious note, taxpayer is 24 on 12/31/24

Is the program not allowing it, or is IRS not allowing it?

For some purposes, your age on December 31 is what you celebrate on your "birthday" of January 1.  For others, at least as far as IRS is concerned, it is not.  See this Revenue Ruling from 2003:

https://www.irs.gov/pub/irs-drop/rr-03-72.pdf 

But that refers to the age of the child who qualifies a taxpayer to claim the credit, not to the age of the taxpayer. 

The rule that "you turn 65 on December 31 if your birthday is January 1" has been around so long, I don't know its source.  I don't think it's in any statute.  

For purposes of the Standard Deduction, he was age 25.

For everything else, he was age 24 (which disqualifies a childless Earned Income Credit).