Just that specifically? Don't know - also not sure how you think linking eternal youth without immortality would work. You just looking to freeze the portrait at some point but continue to age and die?

Have played with a 'supernatural/magic' mod before that institutes a sort of immortality that freezes the portrait at the point the 'spell' is cast - you could see how the code works and use that or run the mod with all the 'extras' but it is pretty extensive with the 'witchcraft'.

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2837844350