When forty winters shall besiege thy brow,
And dig deep trenches in thy beauty's field,
Thy youth's proud livery so gaz'd on now,
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;
To say, within thine own deep sunken eyes,
Were an all-eating shame, and thriftless praise,
People will ask him where his beauty, his youth and all his  metaphorical  ‘treasure’ — a sexual innuendo — are hidden. 
 The reference to “treasure” is the first of many in Shakespeare’s sonnets where gold, or money or wealth is used to describe the beauty of the boy and the loving nature of the speaker’s relationship with him.  A good example is  Sonnet 4 .
Sonnet 2