I've written before that I think teenagers are one of God's top three worst ideas. My wife argues that teenagers aren't one of God's ideas at all, but a creation of society. It used to be you were a kid until you were an adult. Now you're a kid until your parents die (and unfortunately for our national finances, the Baby Boomers just aren't dying like we need them to).
If teenagers are a social fiction, then we can do away with them. Why would we want to? Well, let's begin by addressing the costs they impose on others. A class of non-productive people in constant need of new entertainment is a massive resource drain. Entertainment serves a purpose if it allows a productive person to relax and thereby be more productive with the rest of his time. Relaxing to get ready for more relaxing doesn't produce a damn thing.
Much of what teenagers do for "entertainment" is destructive. Here's a story about a really pretty* girl with big ears who was mercilessly bullied by her peers. Now, it's true that if she could have stuck it out to college, she would have been just fine. No one was going to bully her anymore (assuming she didn't go to a community college, which is just an extension of high school), she would learn how to accentuate her positives, and she would have ended up happy and fulfilled. And if her peers were distracted from her ears by bearing actual cares of the world, she would have made it. But since everyone around her has every day off until they are 18, they had plenty of time to find something different about her and exploit it for "entertainment." And depending on the bullies' intensity and the girl's sensitivity, she maybe wouldn't have lived that long.
Why is there no cost-benefit analysis of prolonged childhood? Surely things like this would go on the "cost" side of the ledger. And where are the benefits? Are modern adults happier than their parents' generation? No. Is labor productivity higher? No. Are educational outcomes higher? No. I don't see any benefits from the modern teenager, but this girl's story highlights many of the costs.
* You might disagree with my assessment that she's really pretty in the "before" picture, but it's fairly obvious her "before" picture isn't her most flattering. She's beautiful in the CNN picture, and that's a result of just hair, makeup, and clothes, not her ears. Superficial things can't make you beautiful, they can just highlight the beauty that's already there.