Quatrain 3 Summary

Get out the microscope, because we’re going through this poem line-by-line.

In me thou seest the glowing of such fire,

Line 9 begins the third major section of the poem, quatrain 3. Are you starting to notice a pattern here?
That's right once again the speaker is telling the person that he's talking to that there's something to be seen in him.
But the similarity between these three lines doesn't end there. Notice also that each one of these three section-openers introduces a different set of imagery to serve as a metaphor for the speaker's situation in life. In line 1 we got the time of year, the dominant metaphor in quatrain 1; in line 5 we got the "twilight of such day," the dominant metaphor in quatrain 2, and now, at the beginning of quatrain 3, we get "the glowing of such fire."
Based on what we've read of the poem so far, line 9 can help us make some predictions about the rest of quatrain 3. From where we're standing now, we can probably assume that quatrain 3 will use fire imagery as a metaphor for aging and death. But hey, that's just Shmoop's guess. What's yours?

That on the ashes of his youth doth lie,

Hmm, now this is a tad unexpected. Doesn't it seem a little bit like Shakespeare is mixing up his metaphors here? Like, sure, the "ashes" bit at the beginning of this line sounds like it goes with the "fire" imagery—but what about the "his youth" part?
It's not that the general idea is unclear—we can tell that "his youth" refers to the earlier fuel for the fire that is now burnt up and reduced to ashes—but it still seems weird that Shakespeare would refer to the glowing of the fire as having a "youth."
It's almost as if, in line 9 and the beginning of line 10 (up to "ashes"), Shakespeare decided to refer to the human speaker of the poem in terms of imagery derived from fires—but then, in the rest of line 10, he doubles back on himself and starts referring to the glowing fire metaphorically in terms derived from human life.
We read this line as personifying fire. No seriously. If fire has a youth, which is when it's young and strong and blazing, then it has to have an old age. That's when a fire's just kind of smoldering, burning up the last chunks of wood, but mostly just reducing to ashes.

As the death-bed whereon it must expire,

In this line, we can see Shakespeare following the same pattern he started in line 10: describing the glowing of the fire (which he originally introduced as a way of illustrating human experience) in human terms.
Here, the last remnant of the fire burning on top of a heap of ashes gets compared to a person on their deathbed.
We don't know about you, but we think the word "must" here conveys a strong sense of the inevitability of death.

Consumed with that which it was nourished by.

This line might seem a little strange on a first reading, but it becomes less so when you remember that (strictly speaking) the "glowing" of the fire, not the fire itself, is the central image of quatrain 3.
The "glowing" was originally "nourished" (i.e. kept in existence) by the wood or whatever it was burning. But as the fire keeps burning, the fuel turns to ash.
Soon enough, the time will come when there is no fuel left. When that happens, it will be safe to say that the "glowing" has been "consumed" (i.e. destroyed) by the same thing (the "fire") that kept it in existence, since the ashes are snuffing out the fire.
Of course, in all of this you have to bear in mind that the glowing fire image isn't just there for its own sake. Based on the poem so far, it's pretty clear that this imagery is being used as a metaphor for human life.
How would you match up line 12 to an idea of human life? We at Shmoop would put it this way: youth has a "fire in its belly" that makes the beginning of life an exciting time. After a while, however, people who lead very active, exciting lives can experience "burnout." Thus, you could say that the very same energy that kept a person going was the same thing that eventually wore them down.
(Notice how, when we use expressions like "fire of youth" and "burnout" in everyday English, we end up using poetic metaphors similar to what Shakespeare uses in his poem? We think that's pretty darn cool.)