The subject matter is what the poem is about, whereas the theme is what the author wants to say about the subject matter; a theme is the direction the exploration of the subject matter takes.  In the case of Longfellow's poem "My Lost Youth", the subject matter is youth.  The theme is the integration of the past and present as concerns the hopes and dreams of youth - the author is saying that the memories and aspirations he held as a young man are still with him today.  The chorus of the poem, which was taken from a Lapland song, reflects this theme of continuity and universality - "A boy's will is the wind's will, and the thoughts of youth are long, long thoughts".

The author carries the theme of the transcendence of the things of youth to the present throughout the poem with the repetition of the word "still".  The poem is set in the seaside city of Portland, Maine, where the boy grew up, and as an adult he visits it still.  Returning to the city, even in his mind, evokes memories of the beauty of the seascape he used to gaze upon when he was young, along with ties to historical events and experiences in his life both joyful and sad.  The city of his youth, and in a larger sense the things that made that time of his life significant and real are still vivid and alive, a huge part of whom the author is today.