Fountain of Youth
From the Super Mario Wiki
The Fountain of Youth is a magical fountain that appears 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, which is due to a lever. During the "Two Plumbers and a Baby" episode, three elderly people try to become eighteen again; however King Goo Goo Ga Ga Koopa pulls 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 return the Fountain of Youth to its original state, and transform the princess to her normal age. They also return a lot of babies back to their old age. King Goo Goo Ga Ga Koopa is then thrown in the Fountain of Youth, and turns into a baby.