Robert Herrick echoes many other poets and playwrights in this poem, in that he praises youth as the best part of one's life. He is talking about it in a particular sense, however. The poem is addressed to "virgins," and he may not be suggesting that youth is better than old age in every sense, but specifically in a sexual sense. The young are generally understood to be more beautiful than older people; Herrick describes this early age as the time when "youth and blood are warmer." He is here suggesting that young people are physically more active and more filled with sexual desire and capacity. Older people are not only less beautiful in traditional terms, they also desire sex less, or at least are (in some cases) less capable of achieving it physically. It is also important that women "marry" in their "prime"—that is, when they are in the prime of their sexual maturity—because otherwise, they will find it more difficult to conceive children, something that was considered the primary goal of marriage in Herrick's time.

We might have different opinions about what constitutes the "prime" of life in this day and age. Certainly, we are less preoccupied with the necessity to procreate as a means of justifying one's own existence, so we no longer necessarily equate fertility with "prime." However, in many ways the modern world does still see youth as the prime of life. You may disagree, however—some might consider the 30s or 40s now the "prime" of life, as this is the age at which people are usually starting to earn more money and reach higher levels of confidence with themselves and their careers, etc.