"Always Seventeen"

I'm on a Harry Chapin kick tonight. This one is my favorite, I think: the instrumentation and the chorus hit me right in the heart:

"Harry Chapin- She Is Always Seventeen" (Watch on YouTube)

Oh, she is always seventeen:
She has a dream that she will lend us and a love that we can borrow,
There is so much joy inside her, she will even share our sorrow,
She's our past, our present, and our promise of tomorrow,
Oh, truly she's the only hope I've seen, and she is always seventeen.

(Lots of possible interpretations. I believe he meant that to him, idealism was always a young woman of seventeen.)