The subtitle of "The Prelude" is "Growth of a Poet's Mind." In this autobiographical poem, Wordsworth details how his outlook has changed over the course of decades, from his carefree boyhood and his idealistic youth, when he was a fervent believer in the French Revolution and radical ideas, to the conservatism of his later years. Yet Wordsworth doesn't present his changed outlook as representing a radical break with the past. On the contrary, he sees his mind's growth as an organic process, with each stage of development related to the one before and after.

Wordsworth makes himself the hero of his own epic, and this epic narrative must cohere. His youthful experiences of nature, rendered in vivid accounts of skating on frozen lakes, flying kites, and playing cards, represent the "seed-time" of his soul. They are not isolated incidents; they point towards the subsequent development of his poetic mind. The growth of this poet's mind emerges naturally out of his earliest experiences, and most particularly his interactions with nature.

The main idea of "Prelude" is that as we grow older we grow more sophisticated in our views about the world.

This poem is autobiographical, and was intended to be Wordsworth's views on life when he was younger and the changes in his views as he got older.

Whate'er its mission, the soft breeze can come

          To none more grateful than to me; escaped

          From the vast city, where I long had pined

          A discontented sojourner: now free,

          Free as a bird to settle where I will.

As a child, the speaker enjoys nature and the simple things in life.  Before religion and education, there was nature.  The young see joy in simple things.  When we get older, we get more sophisticated.  We begin to look beyond ourselves, and beyond the simple pleasures that characterize our youth.