The mirror may change into a lake, but it doesn't seem to age as time passes, unlike the woman who sees herself aging in her reflection in its waters. We don't find out that youth and old age are involved in this poem until the last few lines, but throughout the poem we get hints that time is passing. In the second stanza, we see that the woman in the poem is distressed, but we don't know why until the last two lines, which show that the effects of passing time may be the source of her troubles.
The woman uses her reflection in the lake as a clock, measuring how time passes for her appearance.
The passing of time in this poem upsets the woman, but does not bother the mirror.