Young is signed by taking both open and slightly bent hands and touching your finger tips to your shoulders as you brush your hands up the sides of your chest a couple times.
Young is signed upwards, in the opposite direction as clothes, which is signed downwards, so just make sure you don't get the two signs confused.
Signs like young and new are most clearly explained to your baby by contrasting two opposite things, such as a young student versus an old teacher. It usually takes exposure to old and young in a few different contexts for children to start to generalize the meaning of the word.
Teach your toddler the old sign in conjunction with the sign for young. You can talk about some old persons or animals, and contrast them with their younger versions.