This line is part of the refrain of Longfellow's poem, "My Lost Youth", in which he looks back with fond memories to seaport of Portland, in Maine, where he was born. The m begins thus:-
"Often I think of the beautiful town
That is seated by the sea;
Often in thought go up and down
The pleasant streets of the dear old town,
And my youth comes back to me.
And a verse of a Lapland song
Is haunting my memory still:
'A boy's will is the wind's will,
And the thoughts of youth are long, long thoughts.'" An old man has not much left but memories. His life is nearly done; and but a few years remain. So the future does not much occupy his mind. He dwells a good deal in the past; looking back over the long life he has lived, which seems to have gone so quickly. He thinks of his "lost youth", and of all he dreamed of and meant to do then.
What does the old "Lapland song" mean? A boy lives mainly in the present, and takes short views of life. And he is an erratic creature, moved by sudden and incalculable impulses. One can never know what he will be up to next.
One can no more calculate what he will do or say than you can when and how and where the wind will blow. For "the wind bloweth where it listeth, and thou hearest the sound thereof, but can't not tell whence it cometh, and whither it goeth". So, "a boy's will is the wind's will".
But when the boy grows up to be a young man, he begins to think of his future. He stands on the threshold of his life; and all the years to come stretch away before him to a far off horizon. And what a long life it looks! To a youth of twenty, the forty or fifty or even sixty years he may live seem an eternity. He begins to dream of all he will do and become in that vast period ahead. He dreams of effort and achievement. He will become a famous scholar. He will build up a great business and become a millionaire. He will write great books, or paint great pictures, or compose great music, and earn fame. He will take up politics, and rise to position and power. So "the thoughts of youth are long, long thoughts".