Since the first teen drama debuted in 1990, there’s only been one period of time without an active teen drama.
Active refers to: is the show current? It can be on hiatus. It just can’t be canceled.
In 1990, Beverly Hills 90210 debuted.
In 1998, while it was still on the air, Dawson’s Creek premiered.
But when Dawson’s Creek ended in spring of 2003, BH90210 had ended three years prior and there wasn’t any other teen drama on television.
Until…
A few months later The O.C. started in the late summer and very shortly after that, One Tree Hill began.
The O.C. ended in 2007 but, as we very well know, One Tree Hill has been active all this time.
And if this turns out to be the last season of OTH, there’s Gossip Girl (started in 2007) and 90210 (2008).
Will there be another period when there’s no teen drama left on the air? I hope not but time will tell!
TeenDramaWhore 
At least for network television, teen dramas may become an endangered genre but cable nets like ABC Family, Disney Channel and TeenNick have since become a viable ground for developing youth-oriented dramas, with the latter being even able to bring in Canadian teen soaps to American audience.
I believe that teen dramas will survive in the long-term as long as there is a need for an audience to watch a program that they could easily identify with, the genre will stay put.
TDF! Now that humanity has ’em, I hope we’ll never be without. I read my first Sweet Valley High when I was seven or eight (Number 21, The Runaway) when I was seven or eight, and watched 90210 from the first, albeit boring by today’s standards, episode. I’m in my thirties now and can’t imagine my entertainment landscape without this most addictive genre. A few weeks ago my husband and I were trying to imagine what life will be like for us when we’re in retirement. We were watching some old Party of Fives at the time, and I said, Well, I know I’ll still be watching stuff like this!
Thanks for this fun fact! You do great research.
A bit of a grammatical error, I meant to say: “…as long as there is a need from the audience/viewers to watch a program that they could easily identify with, the genre will stay put.”