I remember watching one night on TV in the mid-90s a color movie with English subtitles about a clinic with a doctor that treated elderly people. I said a movie but it could have been a TV movie (or a short TV series with all episodes put together) because it lasted, I believe, more than two hours. The curious thing is that I remember very little of it. Maybe I fall asleep. Also, it seemed to me that the language was Germanic.
This doctor discovered how to delay the ageing process in humans. The doctor experimented on himself and after a few weeks after everything worked well, the delaying process did stop and it started to reverse: the doctor started to become younger but in a weird manner.
I remember a short scene that astonished me:
I saw the feet of someone (I think it was the doctor) who sat in a wheelchair. One of the feet was normal adult size but the other looked like the foot of a child.
There were other doctors that tried to help. Many of them where scientists and Nobel laureates.
It comes to my mind a recurrent motif in the movie. It was a big fish tank or aquarium, but solely with enormous sea turtles. I don't remember what these turtles had to do with the plot but I imagine they were a representation of longevity.
I vaguely remember (this can be another spoiler) that one of the attempts to counteract the catastrophic reversing of the aging of the doctor involved the use of brain cells, from boys that suffered Progeria disorder, brought from South America (I think it was from Brazil).