Amending a prior year return for EITC - what rules apply?

[tngrl]

So, my son's fiance did her own taxes last year (her 2021 taxes).  She has never done taxes before and she didn't make a lot.  She was 23 at the end of 2021.  I need to double check but I am pretty sure she would have qualified for the earned income tax credit at the time.  She just had no clue such a thing existed.  In 2021, you only had to be 19 to qualify if you were single and did not have a qualifying child dependent.  However, the rules changed in 2022, and now you have to be at least 25.  So, can an amended return still be filed to claim the credit she missed on her 2021 taxes?  Do the rules at the time you are filing for apply or the current rules apply when it comes to qualifying for a credit?  

[DoninGA]

The rules that were in affect for the year of the amended return apply.