I shall be telling this with a sigh
Somewhere ages and ages hence:
Two roads diverged in a wood, and I--
I took the one less traveled by,
And that has made all the difference.
Frost himself is the speaker of the poem, as he frequently acknowledged. In the first line of the last stanza he says he shall be telling about his choice of roads with a sigh. The repetition of the word "I" separated by a dash and by the end of a line is intended to represent that sigh. The reader should feel that the poet is stopping, taking a deep breath, letting it out with a sigh, possibly a sigh of regret, and then continuing with "I took the one less traveled by..."
A greater mystery in the last stanza is what he means when he says he shall be telling about his choice "ages and ages hence." This sounds like centuries. Does Frost believe in personal immortality? Or is he only suggesting that his choice may have affected others' lives for generations? For example, if he had chosen the other road he might have married a different woman and had different children, different grandchildren, and so on.
The "sigh" may not be a sigh of regret but a sigh at the contemplation of the enormous possible consequences of simple choices in one's life.