Sonnet 2 bounces back and forth in time, taking us from the present to forty years into the future and back again. This imaginary time travel allows us see the whole range of a man's life, and makes a contrast between warm youth and beauty and coldness, age, and ugliness. Time is definitely tied to the theme of old age, but we want you to notice what a big deal this poem makes about traveling through time.
This poem has a startling impact in part because it's such a brief, concise statement. The poet is able to pack a huge span of time into a few short lines, and this contrast increases the emotional impact of the poem.