how did the old people suddenly become young

A scientist has invented a serum that transforms the old into young and the old into new. His first subject – a very old man – gets a single injection and gradually turns into a baby. Meanwhile, three little men have stolen the syringe with the rejuvenation treatment, which they now give to men and livestock in dire need of it. (stumfilm.dk)

Shouzou and Ine Saitou have been happily married for as long as they can remember. Even in their old age with wrinkles and cracking limbs, the strength of this love is evident in their precious bond. And then, one day, they wake up to find they are young again! Despite their newfound youthful and attractive bodies, Shouzou and Ine remain the same as ever.

One sip from the fountain of youth and old age disappears as you gain a new youthful vigor. This film animation weaves a story of two of our aging population as, in their delirium, they search for their lost youth.

An old geriatrics professor accidentally discovers after a high-school reunion a potion that can rejuvenate anybody and uses it on him and his friends. They all go their new youthful ways but, for most of them, things don't go according to plan.