The Twlight Zone 1959 s5 e11 A Short Drink From A Certain Fountain

Harmon Gordon, a wealthy old man married to a much younger woman named Flora, is exhausted by his wife's youthful and selfish lifestyle. Seeking to keep up the pace, he asks his scientist brother Raymond to inject him with an experimental youth serum. Raymond firmly refuses at first, saying that the serum has had mixed results with laboratory animals and will not be ready for human trials without decades of refinement. Moreover, he abhors Flora for her callous treatment of his brother and is not enthusiastic about any step to strengthen their marriage. When Harmon suggests he will commit suicide rather than lose Flora, Raymond reluctantly agrees to administer the serum.

Per his instructions, Harmon rests after taking the serum. He later wakens to find himself a young man, to Flora's surprised delight. Before Harmon can enjoy his new youth however, the regression continues until he eventually becomes a toddler hours later. Flora tries to leave, but Raymond insists she must stay and raise the infant Harmon or be cut off from Harmon's fortune, threatening legal action against her should she disobey. Finding a stroke of poetic justice in what has happened, Raymond points out that by the time Harmon has regained adulthood, his position with Flora will be reversed, with her being old.