Fountain of Youth
From the Super Mario Wiki
The Fountain of Youth was a magical fountain that appeared in The Super Mario Bros. Super Show! episode "Two Plumbers and a Baby". It is located in the Kingdom of Youth. The Fountain of Youth can make any person any age they want. There is a lever near the pool that can change the age the Fountain of Youth will turn people into. During the "Two Plumbers and a Baby" episode, three elderly people tried to become eighteen again; however King Goo Goo Ga Ga Koopa pulled the lever and made the old men turn into babies instead. Princess Toadstool then fell in the Fountain of Youth and was turned into a baby version of herself.
Mario and Luigi later returned the Fountain of Youth to its original state, and transformed the princess to her normal age. Also, the brothers returned a lot of babies back to their old age. King Goo Goo Ga Ga Koopa was then thrown in the Fountain of Youth and turned into a baby.