Terry Jacks: Seasons In the Sun
Okay, I'm being brutally honest here. I was 10. This song made me weep like a... well, like a 10 year old. Later on I discovered the Beatles, Brian Eno, Bob Dylan, Abba, Stevie Wonder, Pink Floyd, but it was Terry Jacks that got me interested in picking up my dad's guitar.
Jim this song hit me hard too - it's indelible. Even hearing it now is time travel to a time before I understood what music could be and do - but this song was an early, very emotional, am radio based clue.