Think of someone who did something that dominated the headlines when you were about six years old. They showed up again on the news when you were about nine years old. Now think about how much you would remember about them if they popped back up when you were sixteen.
Obviously, it's tough to find direct analogues to Osama bin Laden. The best I could do on short notice was Manuel Noriega. If in, say, the late '90s, Noriega had escaped from prison, I'm pretty sure my reaction would have fallen somewhere between "oh I remember seeing him on the front page of the newspaper once" to "who the f*** is Manuel Noreiega?" So, no, I'm not surprised that a decent number of teenagers have little no idea who Osama bin Laden is. Most kids don't pay much attention to the news at all, and certainly not before they hit their teens, and bin Laden just hasn't been much of a force in the U.S. news since the 2004 election. Remember, Nirvana is on classic rock stations these days, so don't go griping about how the kids don't have the same frame of reference for the world as you do.