I can't remember what the first Sondheim song I ever heard was. It was probably something from West Side Story, or maybe Ethel singing "Ev'rything's Coming Up Roses" on a Lucy Show rerun. I grew up loving Comedy Tonight because of A Funny Thing ... and because King's Island amusement park used it as their jingle.
Yet when I first heard A Weekend in the Country and the rest of A Little Night Music, something changed. It wasn't like anything else I'd ever heard. Fantastic waltzes, hilarious or poignant lyrics. I suddenly wanted to read The Red and the Black, hear the Bartered Bride, learn more about tiny Titians, all to get a better feeling of the time period. I think I was 12 years old at the time, the same age that BK mentioned he was. I learned what perpetual anticipation and liaisons were (and how to spell them).
The ripples spread from there. Soon I was watching Bergman movies, and Smiles of a Summer's Night remains a favorite to this day. More than that, I learned Company and Follies, the rest of his songs and writings.
And I learned a lot about many other things, too many to mention here.