Why can the reader infer that Sir Kay is a young knight with little experience?

Kay is Wart's foster brother and is not nearly as nice of a young man. He is peevish, spoilt and manipulative and attempts to take credit for pulling the sword out of the stone, but fortunately this pretense is discovered and the country is saved from a Kay-led monarchy. Although his heart is basically good his character is nasty and filled with jealousy. His immaturity leads us to think he is young and ignorant.