Two short scenes show what it feels like to grow old, and neither one needs much said. In the first, an old man opens a book and finds lines he once underscored. He can't remember why. The single word "Stops" gets its own line, so we feel him pause. The poem ends on a question that has no answer, because the younger person who made those marks is gone, and the old man can only guess at what mattered to him then.
The second scene shifts from one person to a pair. An old couple "measures time against each other," which could mean they judge their years by what the other has lived through, or that each watches the other's aging as a clock. The speaker doesn't say what to feel, but the tone is quiet, tender and a little sad. Time here isn't dramatic. It shows up in small lapses of memory and in the company of someone who has grown old beside you.