I like to think some boy's been swinging them (Line 3)
The swinging is only done by the boy. Why is it that the older man doesn't start to swing on the tree like he did in his youth? Is it because he is afraid of risking injury, or might there be another reason for avoiding swinging?
Like girls on hands and knees that throw their hair (Line 19)
The girls seem as carefree as the boy. And yet, there seems to be a gender difference in terms of how the girls and the boy interact with the natural world. How do you understand the relationship of each to nature?
I should prefer some boy to bend them (Line 23)
Nowadays, one might think it's strange to take pleasure in the thought of a boy breaking a tree. But Frost takes innocent delight in the idea of a boy swinging on a birch tree. (We later learn that he might be reminiscing about his childhood here.)
Whose only play was what he found himself, (Line 26)
How many boys could live such an isolated life these days? In some parts of the word, this lifestyle is still possible. But most people might have a hard time relating to the life that Frost describes.
One by one he subdued his father's trees (Line 28)
The boy seem to enjoy conquering his father's trees. Is this different from the way an adult would appreciate a birch tree?
[…] He learned all there was To learn about not launching out too soon (lines 32-33)
This would be the best and worst school ever. You'd certainly learn a lot of lessons at once if you fell out of the tree, but it doesn't seem like it would be the most comfortable way of gaining knowledge.