I'm interesting in this regard as I really didn't start listening to music until I was 17 or 18. Up until that point the only music I knew was classical, due to being in orchestra. Also, I don't know why, but I think at one point I got the impression (when I was MUCH younger that my mom didn't want me to listen to alternative stuff (including metal). Turns out, when I was 17 and started listening and started grabbing discs, she said that was never true. So yeah I was a late bloomer. As far as my start, some may laugh, but I credit Staind significantly on a personal level as it helped me through some tough times. Also during that time, Red Hot Chili Peppers really made an impression.
Later in hs in senior year, this is going to sound silly, but my science teacher did some experiment and was talking about metals and shit, and so she was playing AC/DC's High Voltage for me and that was the start of my love for classic rock, despite her calling it metal.
From there though, I think in post high school, that got me curious about some of the older bands and just on a complete and utter whim, I purchased the Black Sabbath compilation "We Sold Our Soul For Rock and Roll", which kickstarted my complete love for metal, which resulted in me going to my first concert which was I think the 2004 Ozzfest where they and Priest shared the stage. I was so entranced by Priest that I immediately got their discography, and from there it just was a whirlwind where I started getting really into the classic metal stuff.
Looking into the classic metal stuff though, I decided to branch out a bit and looked into Helloween's "Keeper" albums, which is what ultimately started to get me interested in the underground metal stuff, and as they say the rest is history.
So basically:
Staind and RHCP - got me interested in music on a personal level.
AC/DC - Complete accident due to my Senior year high school teacher which led me to...
Black Sabbath - Got me interested in classic metal, which led to...
Judas Priest - Which got me interested into such an array of metal, especially the heavier stuff and that led to...
Helloween - Got me interested in the underground heavy metal scene.