Classically Modern
At first, the lack of rhyme and meditative style make this poem sound pretty modern, as we find in the first line, "Children are dumb to say how hot the day is." But it's not long before we can feel this thing settling into a classic iambic pentameter, which we can already feel in line 2, "How hot the scent is of the summer rose." On top of all that, the soft sounds of words like "rose," "summer," and "dumb" give the poem a soft, humming quality through the consonance of the S and M sounds. You're not hearing words like "crack" or "attack" in these lines. Rather, they sound pretty calm and thoughtful in our mind's ear (if you can picture that).
As the poem continues, we start to hear Graves' heroic couplets, and at this point we can realize that we're listening to or reading a poem that's very meditative and classically in-tune, even as it has just enough non-rhyming to remind us that we're not reading some old stalwart like Alexander Pope. All in all, we can feel that we're listening to someone with experience of the world, who is now reflecting—both on a content level and on a sonic one—on what it means to be an adult caught up in the world of language (and sound).