A teen who listens to New Kids on the Block would be retro, not hip.
But a story that can reference Lady Gaga and the newest hottest TV shows, is going to sound hip and trendy and who doesn't want that in dealing with teens.
The problem of course is that generally several years will pass between the time a novel is first written and the time it is available for sale. That teen star that everyone loved when you started writing might be on their third trip to rehab by the time the book comes out.
What's an author to do?
Some people (and perhaps they know they will have shorter leave time, or are tapped into the newest trends that will be around awhile) are able to pull off the hip and trendy. I can't do that. I wasn't particularly sensitive to trends when I was a teen, and that really hasn't improved over time. And my kids are seriously retro, (seriously: Calvin and Hobbs, Peanuts, Archie, 40s Jazz, Beatles, Broadway music, M*A*S*H and Gilligan's Island) so I can't get clues from what they are watching or listening to.
If I use a real band or book or person as a reference when I write I try to make it be someone who is so "out" as to be "in" again. People or things that have already stood the test of time and aren't likely to fade into complete oblivion anytime soon.
But for the most part, I make up my own pop culture for my books. The songs or stars they are talking about are of my own invention. Sure, they won't be immediately recognizable to anyone, but they also won't have done a stint in rehab.
I think it is impossible to be truly timeless. Something is always going to date the manuscript. Technology is a big one. I got my first cell phone only eleven years ago, but kids these days can't imagine not having them. TiVo and other services mean that you don't have to watch a particular show at a particular time anymore. Netflix means people aren't running out to rent movies at a store someplace. And who knows what other changes will be in store in the next couple of years.
Even political or geographical things don't stay the same. I risk dating myself with this example, but when I was a teenager I wrote a story where the main characters visited the Berlin Wall. The Berlin Wall had been a constant since before I was born. I couldn't imagine it ever not existing. But if I ever wanted to keep that scene in a story, it would have to be historical now, since the Berlin Wall is long gone.
So, when it comes right down to it I guess it's impossible not to date your story in some way. What are some ways you try to work around that?