Maybe you clever folk will be more ingenious than I've been.
The story goes like this:
An old king's life is upended one day when a beautiful, mysterious woman appears and says she'll grant youth and her hand in marriage to the man that completes some challenges. The only challenge I remember was that she sets up 3 cauldrons: One had boiling oil. One had milk. The last had ice-cold water.
The king wants to see it work first so he points to a random little boy and orders him to jump into the cauldrons or he'd be put to death.
The boy leaps into each cauldron and there's a terrible delay on the last one. The cold water cauldron even freezes over.
The boy breaks out of the last cauldron and has been transformed into a strapping young man.
The king, seeing proof that it works, decides to jump into the cauldrons. However, when he hops out, he's still an old man.
The woman announces that the magic only works once, and she and the stable boy walk away together, arm-in-arm.
...
I've searched for it online a fair bit but I've never found it.
Some details from my memory:
* The cartoon was very short (less than 30 minutes. probably closer to 10 or 15)
* It had no dialog, only sound effects and music.
* A woman's voice narrated it. I can still hear her.
* Now that I'm grown, I see it having a Slavic or Russian aesthetic.
* The woman had black hair and a long white dress.
* The king was very short with a big white beard.
* The boy, when he turns into a man, has pointy boots and shoulder pads. :)
* Probably made between 1975 and 1985
* Part of an anthology (many cartoons on one VHS tape... ours had been run so much that it started to skew and stretch the image)
In my mind, it's aesthetically very similar to an heirloom that my grandmother made and I assume that's why I've always wanted to find it.
ChatGPT and the intertubes in general haven't been very useful.