Who is sonnet 73 directed towards?

In Sonnet 73, Shakespeare apparently represents himself as being at death's door, when he actually had quite a few good years left in him. Perhaps the point of the poem was to contrast his age with the youth of the person the poem is addressed to, whoever that might be.

However, another dimension is added if one considers the poem to be part of a real-life correspondence between Shakespeare and his patron (for which there is considerable evidence - see The Biography in Shakespeare's Sonnets at related link below). Now, on closer inspection of this poem, we can see that nowhere does the poet actually state that he is old or that he has the characteristics of old age. The closing line is strangely phrased, with the implication that the addressee will be doing the "leaving" - not the poet. Nor is this phrasing a consequence of poetic constraint: Shakespeare could, for example, readily have used the Word "lose" instead of "leave", had he intended to refer to his death.

On this basis, the patron is depicted with the perception that the poet is passé and that their relationship is nearly done: "In me you see things that are past it - like a year in the grips of winter, a day at dusk or a guttering fire". The patron is on the verge of abandoning the poet, who puts in a clever plea to be loved well nevertheless - tacitly in the form of a good redundancy package! A perfect example of Shakespeare's ability to convey hidden or parallel messages in the form of hauntingly beautiful poetry.