For me it was about 9-10 years ago. Early 2002. I was a Freshman in high school and my musical tastes had been changing and expanding. I was growing tired of the same old effortless and souless rock and roll, and hip hop had nothing exciting to offer anymore it seemed. I had begun to hear songs here and there from Clapton, The Who, Rush, etc. I had also heard Zeppelin here and there (just didn't know it at the time). That stuff began to catch my attention more and more. I went out of my way to hear as much older stuff as I could.
My Dad had noticed my interest and shifting music taste. He told me that If I was liking the "classic rock" stuff, that he had an album for me to listen to. That album was Physical Graffiti. He told me that the last song on the first side was the best on the record and to pay close attention to it and tell him what I thought. That song, as we all know, is "Kashmir." The moment I heard the opening drum crash and riff to Kashmir, I was an instant Zep Head and I haven't looked back. That song didn't just wow me. It changed my musical tastes forever! I didn't listen to the whole album that day, but the "Zep seed" had been firmly planted. I couldn't beleive I hadn't been listening to this stiff all my life. It was such a refresher for my ears, and senses. Needless to say, my review of the song to my Dad was beyond glowing.
I then heard "Whole Lotta Love" and "Black Dog" on the radio the following week and I went out and purchsed Early and Latter Days that very day. That was the early inrtoduction to Led Zeppelin for me. After loving every song on there, I went out and began getting all the studio albums I could. I wanted to soak up all their music. It's the best decision I ever made. Their music is timeless, amazing, and still my absolute favorite to this day.
I now own all the albums, many live albums and compilations, and concerts. It's been a fun journey. I guess I owe my Dad a bit of thanks for handing me that masterpiece of an album.