The answer is not to shirk away from objective reality. You can't solve a problem when you can't objectively define it. After all, I could easily argue you are not an adult based on your responses here. But what would that solve? Nothing. Just as you labeling someone over 18 who doesn't believe and act as you would want them do doesn't solve anything. Though it might give you some minor feeling of smugness in not being associated with them as adults, it does nothing objectively to alter the problem of lack of maturity and responsibility.

One of the key problems we have as a society is we isolate children and make this distinction. We stick them in environments they do not have influence in or over until they "get out of school" wherein they are thrust into a world absent the basic ability to be a part of something other than their age and class.

And we then play at being shocked when they can't handle it. Why should they be? They are raised in a peer group that includes only authority figure and people their own age. This lack of true diversity is a prime condition for the problems we see in this video, such as a lack of understanding of groups other than one's own. Notice this has nothing to do with adulthood.

You have your assumptions entirely backward, and your petulance is promoting that, preventing you from seeing the obvious and correct conclusions. Recognizing their status as an adult isn't about giving them permission to be adults, though that can be a side-effect, it is removing the permission to not behave as one. The mind is a slippery thing but there are things we do know about it.

The non-conscious mind has no temporal understanding. To it "someday", "when you are older", and "when you are an adult" do not translate the way they do for our conscious mind. To the non-conscious mind it is now or not now. If you are not "an adult", the non-conscious mind will not push you toward the behaviors we expect as an adult. It will put those "on the shelf" until you are "an adult". As long as you label them as other than an adult, you tell their non-conscious brain, which controls most of our behavior, they don't have to behave as one.

And to your final attempt at wit, no emulating anyone is not a requirement of adulthood. Being born at least 18 years ago is the sole requirement. What you do after that point determines what type of adult you are, not whether you are one or not.

This is similar in many ways to the modern attempt by feminists to rewrite what it means to "be a man" or what "male strength" means. Lacking objective definitions you will always face arbitrarily emotional characterization so. As the saying goes, try to build a bridge without rigid principles. With a subjective meaning to "adulthood", as you are asserting, I can easily define you as not an adult.

Once I have that definition I am than justified in denying you the rights of adults, while you are justified in refusing to meet the responsibilities of one. Much like bridges require rigid principles, if you want to build better adults, you have to have rigid definitions of adulthood, rather than emotional and ideological ones. Even a rickety rope bridge is still a bridge.

Now if all you want to do is continue to incorrectly label adults as non-adults because you don't like them, don't bother responding unless you also have a need to have "the last word". So feel free to have it. If you aren't willing to stick to objective facts and instead insist on ones interpretable by anyone in any way, you aren't worth any further typing on my part. We have more than enough of that outside the Gulch.