The Czech 1977 movie Což takhle dát si špenát can be seen here in quite good quality : https://www.youtube.com/watch?v=MmguPSSKsWc
This comedy include Age regression of two mens, and later one woman mental AR : two thieves steal an invention used for rejuvenating old cows, and end up waking up as little kids after eating spinach (the _title_ mean How about spinach?, a sentence the server ask the two mens around 24 minutes in the video, followed by the wake up scene).
There's no process, and worse of it all the kids keep adult voices, but it's a fun movie. For some reason I don't quite get, a woman around the 1 hour mark wake up still adult, but mentally regressed to a baby with a baby voice, while the two thieves wake up as their old selves, in outgrown cracked boy pajamas. There's also a woman regressed to a baby's body, with her adult mind and voice, although I don't understand at all what's going on, from the lack of sub_title_s. ^^'
What surprised me the most when I discovered this movie some years ago is the ending, which is quite dark for a comedy
Big spoiler :
The two aged back thieves somehow end up shrunk down in size to around ten centimeters, I think because they purposely tried to get younger again to evade arest and it somehow went wrong (something to go with spinach?).
They successfuly escape by hiding in some pastry that a man take out in his bag, only for a dog to eat them, and the mens alive as they scream, the movie ending with the woman wondering what was that noise. Oo