You're still a greenhorn, yet you think you're a full-fledged adult?
青二才 also means 'greenhorn' or 'young punk', but is more literary and less common in modern casual speech. 小僧っ子 is more colloquial and directly insulting.
Standard kanji spelling for this derogatory term.
Less common variant without the small っ.
Formed from 小僧, “youngster; novice,” and the colloquial suffix っ子, producing a familiar or contemptuous term for an inexperienced youth.