What is the main theme depicted in Sylvia Plath's "Mirror"?

This is an excellent poem and one I always give to my AP
English Literature and Composition group to analyse at the beginning of each year. To
put it simply, this poem is about aging and its inevitability. However, what makes this
poem unique is the novel way in which Plath explores this issue by assuming the persona
of a mirror that is looked into by its owner as she follows the process of her aging.
Note what the mirror tells us about its owner:

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She rewards me with tears and an agitation of
hands.
I am important to her. She comes and
goes.

Note how the persona of
the mirror speaks in a detached, curtailed and precise way, using short sentences that
reflect the mirrors self-professed objectivity. It also presents us with the way that,
for some people, we obsess over our appearance so much that we become slaves to our own
reflection and process of aging.

Another aspect of this
tremendous poem is the closing simile that represents old age as a "terrible
fish":

In me
she has drowned a young girl, and in me an old woman
Rises toward her day
after day, like a terrible
fish.

Note here how this poem
comments on such obsession: the danger of focussing so much on our aging is that we,
like the woman in the poem, "drown" ourselves whilst young because we are not able to
enjoy our youth whilst we have it, and we end up living a life dominated by the fear of
the inevitable, pictured in the "terrible fish" of the old woman that she knows she will
become.