Reason for “le” in this exercise sentence?

Karl,  " Le " in the sentence is a 'neuter pronoun',  replacing the adjective 'jeune'. This is covered in the lesson linked below.

Without using the pronoun 'le' the sentence would be 

" Nous paraissons plus jeunes que nous sommes jeunes ". 

Unlike in English, the adjective needs to be 'repeated', but can be replaced by the pronoun ' le ' instead.

 Using neuter pronoun le to replace adjectives (French Direct Object Pronouns)