* Quel is the interrogative adjective for "what" and is used before a masculine noun.
* Âge is the noun for "age" and is masculine.
* As-tu is the informal way to say "have you" in the second person singular.
So, the full phrase translates to "How old are you?"
Quelle would be used for feminine nouns. For example, "Quelle couleur as-tu?" means "What color do you have?"