Will be a tatter'd weed, of small worth held: (line 4)
We think this is a sad and sort of beautiful image of the difficult parts of getting old. Your body starts to feel a little worn, and maybe people don't notice you as much as they used to, or don't value your contributions in quite the same way. Of course this isn't all that old age has to offer, but remember that the poet has to make aging sound bad, or the young man won't do what he wants him to do, which is to have kids.
To say, within thine own deep-sunken eyes, (line 7)
Here's another image of the crummy side of getting old. When we read this we can almost see a skull staring out at us from hollow eye sockets. At this point old age and death seem close together. The poet is reminding his reader that not only will he get old, he will eventually be completely gone, unless he leaves behind a son to keep his beauty alive in the world.
And see thy blood warm when thou feel'st it cold. (line 14)
In this poem, old age means withering away, growing cold, becoming tired and tattered. Resisting that change is old theme in literature. It's only human to look for ways to escape from death, to hold it off for a while, to cheat it. What the speaker of the poem is offering here is something like a solution to that problem, a way to escape from death. According to the speaker, when you have a child, you continue yourself, you take the cold and make it warm again, and you take the old and make it young. Of course, having children won't keep you from dying, but it will give you a certain way of continuing your life.