Complex sentence: He points out the essential inconsistency in saying that the poet was young and then expecting maturity in his early work before then going on to point out that for him the key point is not whether a poem is mature or not but whether it has artistic merit.
Split into simpler sentences: He points out the essential inconsistency in saying that the poet was young and then expecting maturity in his early work. He then states that for him the key factor is not whether a poem is mature or not but whether it has artistic merit.