The speaker of Shakespeare's sonnet 73 seems to be the same speaker as the surrounding sonnets, and the addressee, too, is generally supposed to be the same. The sequence of sonnets to 1-126 constitute what scholars call the "Fair Youth" sequence. Sonnet 73 is among the most famous. The speaker spends 12 of the sonnet's characteristic 14 lines laming his old age ("In me thou see'st the glowing of such fire / That on the ashes of his youth doth lie"). The final two lines overtly address the love object. Shakespeare writes:
This thou perceivest, which makes thy love more strong,
To love that well which thou must leave ere long.
The neatest understanding of these is that Shakespeare was writing to a male, owing to gender designation in other sonnets. The possibility therefore exists that Shakespeare was writing to a homosexual love interest who is younger than the speaker. While by modern standards this option is unlikely, such was wholly unheard of in Shakespeare's era. Modern and pre-modern England was steeped in the classics, which featured forms of "pederasty" (love between a more mature, older man and a fair, youthful man).
Within this sonnet, it's only possible to tell that the speaker appreciates the addressee's love for him. The best analog for understanding his addressee as a man is Sonnet 20, which reads:
A woman’s face with Nature’s own hand painted
Hast thou, the master-mistress of my passion.
Thus, here and elsewhere, while it's impossible to know for sure, it seems that Shakespeare is addressing a young man, whom he loves romantically.