That age is best which is the first, When youth and blood are warmer; (9-10)
There are a lot of words here that have to do with time: "age," "first," and "youth." It's strange that the best time of life is when "youth and blood are warmer," because the previous stanza suggests that the higher the sun gets – the sun gets "warmer" as it progresses, up to a certain point – the closer it is to setting. The metaphors in the poem seem to trip over each other.
But being spent, the worse, and worst Times still succeed the former. (11-12)
These lines pick up the themes of lines 5 to 8. In the same way that the
sun's progress brings it one step closer to "setting," so too does the
end of youth bring "worse, and worst / Times," a metaphorical "setting"
of sorts. Throughout the poem, but especially in these lines, the poem
doesn't seem to place any value on aging.
Then be not coy, but use your time, And while ye may, go marry: (13-14)
The speaker reiterates the carpe diem (or "seize the day") theme from the first line, explicitly describing it as a proper use of time. There might be some wordplay going on with that word "marry." The speaker encourages the virgins to get married, but he might also expect us to hear the word "merry," as if getting married were also a way of making oneself "merry."