I’ve toyed with this concept myself, see the Mantle of Immortality I’ve created. I know this is not exactly what they’re looking for though, as getting young would take as long as getting old with how i created it.
Feel free to change and use it if you like it!
I really like your idea on how to achieve this! Personally I wouldn’t make it harder for them to resist the demon immediatly. Consequences for completing the ritual would be appropriate though. Maybe consider noone finding out because they start aging backwards slowly at first, until after a week or two the party notices them looking younger and start asking questions. You know, put it as a story element rather than a mechanical one.
That’s just my two copper pieces.
EDIT:
I’ll add the link to the item in a few minutes when I’m not on mobile anymore.
EDIT 2:
There we go: Vrenshrrg's Extraordinary Items