Monkey Business (1952)

Barnaby Fulton is a research chemist working on a fountain of youth pill for a chemical company. While trying a sample dose on himself, he accidentally gets a dose of a mixture added to the water cooler and believes his potion is what is working. The mixture temporarily causes him to feel and act like a teenager, including correcting his vision. When his wife gets a dose that is even larger, she regresses even further into her childhood. When an old boyfriend meets her in this state, he believes that her never wanting to see him again means a divorce and a chance for him.