You’d have to create a separate character for the young version of the customised characters.
Then you’d have to edit the customisation script like this:
@MAINCHARACTER changes eyesColor into Violet @YOUNGCHARACTER changes eyesColor into Violet
So that every choice they reader may pick, the same customisation happens to the younger version. It’s then up to you to choose what the reader can / cannot customise.
You may not want them to change the hairstyle, and that is what the difference is between the young and adult character. So you’d leave the @YOUNGCHARACTER out of that part of the script.