There are a few ways in which the stucture of Frost's poem nods at its theme. First, there is the most obvious answer. Just hold the poem at arms length and take a look at the line length. You should notice that the poem is tall and skinny -- like a birch. Voila! The next structural thing I would point out is the meter of the poem. "Birches" is written in what is pretty standard Frost iambic blank verse, which means that, although it doesn't rhyme, there is a pretty predictable pattern to the language. The reason that this is important is that we can think of climbing birches, in a metaphorical sense, as aging or leaving youth. Although the exact path that this speaker (or anyone) takes out of their youth is unpredictable (i.e. it doesn't rhyme), the same parameters of the journey are the same (i.e. the 10-syllable lines in iambic pentameter). The meter of the poem echoes one of the themes: the loss of youth, though perhaps unpredicatble to some, is inevitable for all.