。
She's young, but she handled the job.
Literal
She [topic-は] young [although-けれども] that work [object-を] handled.
けれども is the full, more formal form of けど (but/although). こなす means to handle skillfully, to manage, or to get through — it implies competence and successful completion despite difficulty. The contrast set up by けれども is between her youth (implying inexperience) and her successful handling of the work. こなす is a versatile verb that adds a nuance of competent mastery.