I answered “Age 13,” but I guess it was in 1983, when I turned 15, that I really turned on to popular music: buying albums on cassette (and asking for them for Christmas); listening to rock radio and pushing the Record button on my boombox when a good song came on; staying up on Friday and Saturday nights to watch videos on WTBS’s Night Tracks (this was before the local cable carried MTV, which is why you were supposed to “Call your local cable company and tell them ‘I want my MTV!’”).
About 9, mid-70s. My school class was a combined 4th/5th/6th thing where we had a fair bit of freedom, and every Friday we’d have a little party with music and games. People would bring their singles to play and I got to like artists like Elton John, Led Zeppelin, Queen, the Doobie Brothers, and lots more. I also had a radio that I’d listen to a lot in my bedroom, so I was pretty familiar with most of the top-40 type music from the era. I mostly bought music on 45 singles, and would ask for albums for birthdays/Christmas etc. and save up to buy the occasional one on my own.
I remember two really frustrating things from that era that “the kids today” don’t have to deal with: how hard it was to find a 45 if it wasn’t at your local record store (for example, my holy grail as a kid was the Moody Blues’ “Nights in White Satin,” which was already kind of an oldie by the time I went looking for it, and I was so happy when I finally found it in an oldies bin when we were on a family trip), and the near-impossibility of finding lyrics to anything if the artist didn’t print them on the album sleeve (which most of them didn’t).
I got a portable record player for my birthday when I was 10 (1970), along with 3 LP records chosen by my Mom. I remember thinking that I wouldn’t have chosen any of them, though I got fond of the ‘5th Dimension’s Greatest Hits’, so I must have had opinions on music before then.
By the time I was 11, I was saving up my allowance to buy 45s of my own choice once a week. It meant cutting back on icecreams from the neighborhood truck, so quite a sacrifice 
It had to be “7 or younger” because when I was a kid, I wanted Captain Fantastic and the Brown Dirt Cowboy for my 8th birthday. And that was nowhere near my first Elton John album.
By the time I was 10, I had a nice collection of about 25 or so 8-tracks.
I put down 10, which would have been 1980. By then I had already horrified my 5th grade class with the “Heavy Metal” of Led Zeppelin, Black Sabbath, and umm, Styx. I place it then because I realized that my classmates had no idea what constituted Rock and Roll, and I had some idea. (For instance, I knew that Styx wasn’t metal.)
However, the moment I became fixated on music was a year or so before that, when I heard “Play With Fire” by the Rolling Stones on the AM oldies station. I’d bought some pop singles, and listened to a bunch of hand-me-down records from my parents. I liked the music, but I couldn’t name any of the artists. That foreboding song where nobody involved seemed happy stood out enough that I remembered who did it, and wanted to find more. Maybe it was because it was different from the happy glurge you’re usually exposed to as a child, maybe I was just a glum kid. It still sets me back on my ass when I listen to it.
Woot! 1200 posts!
I’m gonna say 12 for when I started paying a lot of attention to current music, but I think I had my own favorites before that–it’s just that some of it was inherited albums.
I’ll say 10 (1983) because that’s when I discovered Top 40 music and VIDEOS! Friday Night Videos, Toronto Rocks, Video Hits, Flipside, etc.
I didn’t know popular music before that because my parents never listened to it. Never had a radio playing in the house or in the car. They just weren’t into it.
I was 10 years old in fifth grade, when my academics really started falling apart, and consequently is when I really had to start buckling down and studying. A teacher helped me put together a system whereby I got ahead on the weekends for assignments that I knew were coming (vocabulary every Tuesday, that kind of thing).
So, that’s when I started doing 4-5 hours of homework every Sunday afternoon. Casey Kasem kept me company through that time, educating me about Top 40 radio in a way I’d never been educated before.
When I was 6, for Christmas 1986, my parents gave me an Emerson stereo (AM/FM/cassette/record player) for my room, along with a cassette copy of Bon Jovi’s “Slippery When Wet,” which was the hot album at the time.
I listened the hell out of that tape, and also began raiding my parents’ record collection for the stuff I found interesting, like the KISS live albums and Queen and Cheech & Chong records – I subsequently had a deep understanding of marijuana humor as a young kid.
A few years later, by age 10 or so, I was listening to whatever popular music everyone else was into…top-40 stuff. Lots of crap like Mariah Carey was on my radar…a bit of a dark time for me musically. By about age 11 or 12, I also gained a side interest in gangsta rap like NWA and Eazy E, and I got a copy of Nirvana’s “Nevermind” when it was pretty new, sparking an interest in grunge.
By age 13, I had a CD player and a copy of The Doors’ greatest hits two-disc set, and I was following the grunge/alternative scene. I started playing guitar, so I was into Hendrix and Clapton and lots of other classic rock. To this day, I listen to more 60s and 70s stuff than anything. I somehow didn’t get into the Beatles until college, and now I have hundreds of Beatles bootlegs.
I put down “7 or earlier”. Admittedly my Mom listened only to the local country station and we didn’t have many other choices (we got our first FM radio in 1982 when I was seven), but I did legitimately like the music and listened to that station even when Mom wasn’t listening. We didn’t have enough money to buy records/tapes, but I definately had fave artists and songs. I saw enough of the big name artists on TV to know what they looked like. One thing I do especially remember is that I didn’t really dislike any artists or songs; that would wait until I got that FM radio and started listening to top 40.
I bought my first singles (“Snoopy Vs. the Red Baron” and “I’m a Believer”) in late 1966, when I was 11, so I know I had started listening to the radio on my own at that time. However, I’m not sure I was paying enough attention to meet the criterion of being “knowledgeable”. I do remember that, by early 1969 (when I was 13), I was eagerly awaiting the new Beatles singles, so I’m pretty sure that popular music had become a focus for me by then. I split the difference and voted “12”.