Youth, like death, is a constant backdrop for many of Frost's poems. The speaker of "Birches" never sees a boy or comes across one. He only imagines one, and the boy that he does imagine is himself at a younger age. The boy seems to be similar to William Wordsworth and Walt Whitman's portrayals of boys. These boys have their own rules and wisdom that they can pass on to the older men and women around them. They are ready for adventures in nature and represent the wild, untamed state of "man" that remains good and moral even though no one is there to govern him.
Life as a youth is just as much as a pathless wood as in adulthood; it's the lack of considerations that makes youth easier.
The boy in "Birches" is the teacher to the speaker.