Honestly, I don't think that teenagers are any stupider now than they were in my time (born in '83). We had dangerous, embarrassing, and stupid behaviors galore. What amuses me is that the young think that their own culture of inanity is unique to them ("no one understands me") and that adults don't understand their code words (many of which were invented by or before us).
For example, I'm surprised that people (young and old) seem to think that "the choking game" is new. It's ancient. It's also not appealing to adults because, after you've had a life-threatening health problem (which most of us either have had, or know someone who has had) you're just not into fainting and light-headedness. Signs that your body is malfunctioning are scary. They mean hospitals and medication and possibly death.
The one thing that seems to be universal is that the young think that the culture of stupidity is unique to them, because adults ignore it and politely pretend not to understand it. (Yes, we know what "Molly" is. Drugs weren't invented ten years ago. We, collectively at least, probably did more of them than you.) I certainly wouldn't tell a teenager all of the dumb things I did when I was younger, not because I'm a square who wouldn't want them to have fun, but because I wouldn't want to encourage them with a story that is funny in retrospect (10+ years after the fact) and was stupidly funny at the time, but that was actually quite dangerous.
Once you've had a double-digit number of people close to you actually die, either for reasons entirely not their fault or through freak accidents, your outlook on danger changes. I'm less afraid of death than I used to be, but I'm more respectful toward it and the danger of it. Those "that will never happen to me" illusions fade away fast.
On the whole, I don't think that kids are getting stupider. Remember that idiotic pacifier trend in the 1990s? Or wearing pants with a "droop" to show one's boxers? Or setting one's taint hairs on fire? Nothing is going to beat that stuff in terms of sheer idiocy.
One thing that I like about the current generation of kids is that they seem to be more accepting of gay and transgender people than we were, as teenagers. In the 1990s, I remember that quite a number of people found it socially acceptable to say that AIDS was God punishing people for "deviant" behaviors. It seems a lot rarer to hear that kind of sentiment, and I'm glad.