1.allowing for his age是个短语而已,不是句子啊,even是一个副词,放在它的前面,意思是“即使”.Even allowing for his age只是个短语而已,不能看成是句子.he still acts very immaturely才是个完整的句子；he 是主语,still是副词,acts 是谓语,very immaturely是副词做状语,主干是he acts immaturely.
2.Although 是连词,要连接完整的句子,但是allowing for his age只是个短语,非句子,所以只能选择even这个副词来连接它.