MTV absolutely did not cater just to teens in the 1980's. Actually, it was aimed at the 20-something market and mostly played hard rock and heavy metal at the beginning.
Up until 1983, it was very rare to ever even see a black artist on the channel, and there were no black artists getting anywhere near to heavy airplay.
This changed when Walter Yetnikoff (the head of CBS Records) basically told MTV that he would pull every single video from artists on his label (which was a significant amount), if they did not play the video to Michael Jackson's "Billie Jean". MTV gave in and began rotating the video heavily, and that's the point where the "Thriller" album blew up. Hot on the heels of "Billie Jean" was the video to Prince's "Little Red Corvette", from the "1999" record (same song quoted in my sig lol), and so Prince and MJ began their MTV domination that would last for the rest of the decade, alongside their white peers like Madonna and Springsteen. As Preach mentioned - towards the end of the decade MTV began to cover the hip-hop movement with great shows like Yo! MTV Raps.
In the early 90's, MTV began to aim more at the college market. The 18-25 age range, with shows like "Beavis & Butthead" and music shows like "Headbangers Ball", a show dedicated to the emerging grunge and alternative rock/metal movement.
I would personally argue that MTV did not focus on the "teen market", as we know it now, until 1998. This was when they took focus off edgier stuff, and started a show called TRL (Total Request Live). By 1999, MTV was largely dominated by Backstreet Boys, N'Sync, Britney Spears, Christina Aguilera, type artists.
This was a calculated move by MTV, as by this time, they had shifted alternative music to MTV2, and were branching out into more channels devoted to specific genres. But of course the main advertising income was still coming from the main MTV Channel.
The internet and emerging broadband speeds circa 01/02 really had a big effect on MTV, as people began to download music videos on P2P networks. YouTube was not invented until 2005, but it wasn't exactly difficult to find, watch and share music videos online before that, although it certainly wasn't as simple as it is now. Many of the music videos shared online were rips from MTV with the MTV watermark very obvious. I remember downloading hundreds of music videos online in 2002 and 2003, burning as many as could fit on a blank CD, and watching them in my cousins college dorm rooms, getting high.
MTV, not knowing what to do with their slipping ratings, began to commission more reality shows. Music videos, which in 2000 had represented 8 hours a day of MTV's airtime, rapidly began disappearing from the channel. By 2008 they represented just 3 hours a day of airtime.
MTV had first started doing "reality" shows in the mid 90's with shows like The Real World. By the end of the 90's, you also had prank shows like the Tom Green show and Jackass. But it was the triple-whammy of The Osbornes, Pimp My Ride and Punk'd in 2002-2003 that really boosted rating and convinced MTV execs to continue making shows in this format, as well as shows about celebrities like the always-popular Cribs (which had started in 2000), and things like "Nick & Jessica Newlyweds".
By 2005, you had shows like Laguna Beach, which was superceded by The Hills a couple of years later, and spin-offs like The City, etc. Those shows, plus shows like Super Sweet 16 were the focus of MTV programming for a few years, up until last year, when the backlash against the "scripted" nature of those shows was at an all-time high.
Enter MTV's biggest ratings hit in YEARS.
Jersey Shore.
For my money - it's the best of ALL the reality shows MTV have done. It's just hilariously entertaining. It's not pretentious like "The Hills" and "Laguna Beach" were. It's normal 20-years old going on holiday, getting drunk and trying to hook up. Everyone knows people like the people on the show. And of course, in every group of people there's always a character. In this case, the breakout star is Mike "The Situation" Sorrentino, who stands to make about $5 million dollars in endorsements within the next 12 months and is about to appear on Dancing With the Stars.
Even though MTV is pretty much just another evil fucking corporation these days, I have a soft spot for them. I like to see them doing well. I remember buying MTV branded merchandise at primary school and everybody thinking it was so cool. I would draw the MTV logo all over my exercise books.
And since my dad didn't want us to have cable, whenever I would go to my cousins house I would watch MTV for hours on end. This was in the early 90's when the format was still mainly music videos. Where do you think my excessive knowledge of music comes from? In some ways, I appreciate that format more than how we find music these days. I discovered so much music that I wouldn't have otherwise, through MTV, because you had no choice on what they played. If I would have had a choice, I would have watched Michael Jackson videos and Beavis & Butthead 24/7, lol.
I remember, at one point in about '92, MTV had an entire "Michael Jackson Day". My cousins taped 6 hours of it on a long-play VHS tape for me. You guys have no idea how many times I watched that tape. Over, and over and over again. They had all the great MJ videos, rarer J5/Jacksons stuff, and even a documentary consisting off backstage footage from the "Dangerous" tour, interviewing the roadies, the band, the backing singers, the dancers, fans, and showing footage from the tour.
Good times.