A metaphor is a literary device for conveying complex images or emotions.
A striking extended metaphor in "When You Are Old" seems to me the personification of deep love and the description of life after death in the third, final, stanza of the poem.
In line 10 the word "Love" is a substitute for the name of the 'voice' of the poem. The implication is that love has been the most important thing in his life. Even when he has fled to the hills, which I interpret as a metaphor for dying, his love endures. His spirit cannot rest because his love has not been reciprocated and so it paces amongst the hills until finally his face is forgotten, fading from view, described as being hidden in the vast constellations of stars in the sky. It seems that even when he has been forgotten his spirit, which is comprised of love, somehow endures in the Universe. But that love is no longer accessible to the beloved one to whom the lines are addressed.