Which lines in this sonnet use symbolism to describe old age?
Sonnet 2
by William Shakespeare

A. When forty winters shall besiege thy brow,
B. And dig deep trenches in thy beauty's field,
Thy youth's proud livery so gazed on now,
C. Will be a totter'd weed of small worth held:
Then being asked, where all thy beauty lies,
Where all the treasure of thy lusty days;
D.To say, within thine own deep sunken eyes,
Were an all-eating shame, and thriftless praise.
E. How much more praise deserv'd thy beauty's use,
If thou couldst answer 'This fair child of mine
F. Shall sum my count, and make my old excuse,'
Proving his beauty by succession thine!
This were to be new made when thou art old,
And see thy blood warm when thou feel'st it cold.

In these first lines, Shakespeare uses "forty winters" to call up the pain and cold of winter, just as if you were aging. Forty winters also means forty years. "shall besiege thy brow" is referring to back in the day, when someone would dig a trench and wait, attacking continuously over a period of time (lay siege). So he is comparing this youths brow, or forehead, to the wall of a castle being attacked over a period by time and old age.

If you dig a trench in a field it will not look the same, even if you fill the dirt back in it will never look like it did once before. Time, age, has dug trenches into "beauty's field" the beauty of youth, and that is something that will never return.

Sorry this answer is late, but I'll put it here for anyone still looking.