I think there isn't one issue, but all of these issues at the same time.
wllm teased stealth_bowman about it, but I think the concept of "It’s an age of instant gratification that we are living in and hunting is about as far from that as you can get." needs more consideration.
This isn't just with teenagers, but with people of all ages. Think of how easy it is to sit and watch a specific movie or tv show, custom content on youtube, deep dive on reddit or launch a video game. It takes almost no effort. It can also be really cheap.
Contrast this with what it takes to go hunting. When I was a kid, and I'm no boomer by far, from September through December I could get off school, grab my bow and hunt deer right out my back door. Yeah, there are still some places like that, but I wasn't living in the middle of nowhere. Those fields and brush I used to hunt are houses now.
Now, for me to go hunt is an average 3 hour drive just to be in the area. I pretty much have to commit to a whole weekend. If I was just getting into it, that would sure be a pain in the butt. Has been for the three people I've shown the ropes. They just don't want to make that time commitment.
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The above example illustrates access, too. Those places I hunted as a kid are gone. Some of them still exist, but you damn sure can't just walk onto them not knowing what a property line means. Nobody cared when I walked through the middle of the "block" back then. Now its all "No Trespassing" and "Hunting Property for Lease."
Now you need to do a lot of research, planning and investment before you can start even trying to hunt.
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"Although I have read many articles, and anecdotally this fits with my experience, that kids are increasingly having to specialize and spend all there time in one activity instead of being able to sample many"
This isn't just kids either. Society as a whole now values being "obsessed with" a very particular thing over being well rounded. Just take a look at Instagram or Youtube. Nobody cares unless you're #sickforit . You've got to be totally invested in becoming the best at one very specific thing or you aren't taken seriously. I don't feel like I have the right words to describe this, but I definitely see it.