What techniques and ideas does Sylvia Plath use to construct the idea of aging in her poem "Mirror"?

First, Plath speaks from the point of view of the mirror itself. The mirror does not lie, as the saying goes, and the mirror attests to this in the first verse:

…I have no preconceptions.
Whatever I see I swallow immediately
Just as it is, unmisted by love or dislike.
I am not cruel, only truthful

So by telling her story from this angle we get a more honest representation of aging, an objective viewpoint uncluttered by personal distress, unbiased by any person’s vanity or dissatisfaction.

In the second verse the mirror compares itself to a lake, suggesting that what it reflects is natural, as natural and normal as the world we live in. At the end of the poem Plath returns to this metaphor, stating about the woman who looks every morning into the mirror, “in me she has drowned a young girl,” and “an old woman” is making her way toward the surface of the lake, “like a terrible fish.” In these lines we see the inevitability of getting older, how our past selves and our future selves are all part of the same body, the same lake, in constant flux and yet fixed. And it is from this objective mirror that we see the woman wringing her hands and crying to herself, saddened at the effect time is having on her appearance.

With the coming of the woman, we see how desperately we cling to our reflections, what our own image means to us – the woman uses the mirror as a vehicle to find “what she really is,” and must escape it, back “to those liars, the candles or the moon,” some source of light that will cast flattering shadows, that will hide imperfections. The woman seeks solace in these things, the mirror conjectures, being as the mirror is in the business of reflecting truth. As we age, the truth of the matter is undeniable, no matter how desperate we are to avoid it. We seek out the lies after being confronted with the truth; we try to hide the fact that we are getting older, even from ourselves. And yet even so, says the mirror, “I see her back, and reflect it faithfully.”  Even so, we cannot escape that from which we turn away.