For those who are familiar with Bob Dylan's song-smithing, you will recall that the line is from "My Back Pages" - written in 1964. It first appeared on the album "Another Side of Bob Dylan" though Dylan never performed it live until 1988.
What does the line mean? "Ah but I was so much older then, I'm younger than that now"? There have been different interpretations. To me it's Dylan looking back on earlier times, earlier songs and saying that he is not as certain of things as he once was. Previously he was the voice of a generation - seen as a protest singer - but now he's entering a new phase in which the world is a forest of complexity. He is not saying that angry protesting is wrong, just that it's not as straightforward as it once appeared.
Another of my favourite songwriters is the Californian - Jackson Browne. Here he is with Joan Osborne, revisiting "My Back Pages" and breathing new life into it in 2008. Together they approach the lyrics with reverence and awareness of their almost mystical reach, paying homage not just to Bob Dylan but to the lines themselves:-