People have often seen "To the Virgins" as a poem that exemplifies carpe diem. That's Latin for "seize the day," a phrase meaning "make the most of the time you have." The poem is about making the most of one's time, but it's also about the passage of time, and the fact that as we get older we change. We become less healthy and vigorous, less "warm" (10) and, eventually, die.
The speaker compares people to flowers and the sun, which suggests that our time is more fleeting than we think.
"To the Virgins" suggests that old age – or whatever we choose to call the time after "youth" – is a terrible thing because one's chances of getting married (and potentially starting a family) significantly diminish.