Final answer:
The poem "On Turning Ten" explores the theme of transitioning from the carefree innocence of childhood into maturity, marked by the loss of childhood imagination and the acceptance of reality.
Step-by-step explanation:
A key theme of the poem "On Turning Ten" by Billy Collins is that growing up means letting go of the carefree innocence of childhood. This theme is expressed through the poet's nostalgic and somewhat bitter tone, reflecting on the imaginative wonder of being a child and the realization that the real world isn't quite as magical. The poet's approach highlights the unavoidable transition into maturity, and the accompanying loss of innocence and imagination associated with becoming older. The transition is approached with a mix of sadness and acceptance that marks the end of childhood's imaginative, carefree existence and the beginning of a more realistic adult perspective.