From Mitchel's wiki:
John Purroy Mitchel (July 19, 1879 – July 6, 1918) was the mayor of New York from 1914 to 1917. At age 34 he was the second-youngest ever; he is sometimes referred to as "The Boy Mayor of New York."
so there's the boy reference.
Also
to which a hunter,
a hundred years from now, may look and despair
Mitchel died in 1918 which is almost 100 years ago- I think here Joanna is speaking about herself/the listener/anyone who looks to the past.
This song is astounding. While the theme (I think) is clarifies in the first line (Ozymandian at best), it doesn't stop there/isn't flat. It goes further- it's life-affirming and terrifying at the same time.