This is an excellent poem exploring the psychology and impact of aging upon a female. However, note the ingenious way in which Plath chooses to write this poem. Instead of writing it from the perspective of the woman who is struggling to accept her aging, she chooses to assume the persona of the mirror into which she spends so much time looking at her physical appearance. This allows us to explore both the character of the woman and the character of the mirror who narrates to us, dispassionately, what it sees.
Note how in the second stanza the mirror compares itself to a lake into which the woman peers:
Now I am a lake. A woman bends over me.
Searching my reaches for what she really is.
This simile becomes powerful when we imagine the woman starring intently into the "lake" trying to work out who she is and trying to establish her identity, which is caught up so much in her physical appearance. It is the last two lines of the poem that give it real strength, however:
In me she has drowned a young girl, and in me an old woman
Rises toward her day after day, like a terrible fish.
This captures the theme of aging in the poem, as it talks of how, by focusing so much on external beauty, the woman has ironically drowned her younger self and now can only look in horror at the "terrible fish" that comes towards her as she sees more evidence of her aging every day. The dangers of obsessing too much about our beauty are thus highlighted. We, too, can actually waste our youth by being fixated on the loss of our looks.