what is the meaning of what cheshier said the food in wonderland can make you older but you will learn in experience

Being older doesn't translate to learning .Only experience garners learning that is practical and meaningful.

The Cheshire Cat is warning Alice about the dangers of becoming an adult. He's telling her that although food can make you grow, only mercy and experience can make you wise. It is after the Cat shows her this lesson that she is able to truly mature.