Two lines are all it takes to show a man growing old. The mill wheel dips into the water, turning and working as it always has, and the miller's arms, which once did the heavy labor, are shrinking and going soft "to dough." The image is neat and a little unsettling: dough is what flour becomes, and flour is what a miller makes. The worker is slowly turning into the thing he produces.
The speaker doesn't announce a feeling, and the poem stays calm and matter-of-fact. Still, the contrast does the work. The wheel keeps going while the man weakens, so time seems steady and indifferent to him. The word "delves" suggests effort and digging, which makes the miller's decline feel all the more bitter. The mood is quiet, stark, and gently sad, with a sense that age wears people down through the very work that defined them.