You are only young once, but you can stay immature indefinitely.

The line plays on the distinction between youth as a biological phase and immaturity as a chosen (or at least indulged) attitude. It wryly suggests that while time inevitably ends youth, people can continue behaving childishly long after they have grown older—sometimes by preference, sometimes by habit. In Nash’s characteristic comic moralizing, the humor comes from treating “immaturity” as something one can deliberately “stay” in, as if it were a comfortable residence. The quip can be read as a gentle admonition about responsibility and self-awareness, but it also carries an affectionate tolerance for human folly: aging is mandatory; growing up is optional.

You’re only young once, but you can be immature forever.