Were amazed (existanto). Imperfect indicative middle, descriptive of their continued and repeated astonishment. Common verb existēmi meaning that they stood out of themselves as if their eyes were bulging out. The boy had a holy thirst for knowledge (Plummer), and he used a boy’s way of learning.
At his understanding (epi tēi sunesei). Based on (epi), the grasp and comprehension from suniēmi, comparing and combining things. Cf. Mar 12:33.
His answers (tais apokrisesin autou). It is not difficult to ask hard questions, but this boy had astounding answers to their questions, revealing his amazing intellectual and spiritual growth.