"lovers," especially the speaker, are "withering" or looking much older.
Nature appears in the second quatrain. She is the protective force that tries to "pluck" the young man away from destructive Time. Nature is trying to prove that she is more powerful than Time.
The shift in this poem occurs in the third quatrain (line 9). The speaker warns the young man to fear Nature, for even Nature cannot defeat Time. In other words, although the beauty of the young man may have lasted much longer than it does for most other people, it eventually will experience the ravages of time. The young man will grow old and lose his beauty. Nature is described as someone who has taken out a loan (suggested by the word "audit") but must eventually pay that loan back. The metaphor indicates that even though Nature borrowed from Time to keep the young man looking beautiful, eventually she will have to pay Time back by giving the young man to Time. Thus, the young man will no longer have any power over time. The situation will be reversed. Time will have power over the young man, and he will be a young man no longer.
Although the speaker does not directly state it, he seems to be implying in this poem that the young man should face the inevitable and find a way to preserve his beauty. In other words, as he had done most explicitly in the first sonnet, the speaker is urging the young man once again to have children.
There is no couplet in this poem. This sonnet is only twelve lines long. The poet, perhaps, wished