As far as adulthood goes:
I do believe it is more of a self-imposed myth than an actual requirement. You gain more responsibilities as you get older, and then you need to be able to handle those, but plenty of people don't either because they don't care or don't see it as a big deal. As long as you can make sure you can stay alive, that's mostly all that's needed really.
The are many ways to live life. Some people do what most others in society would do, others like to do their own thing. I put a lot more focus towards people closer to me, but others may be a lot more oriented towards the attitudes of their more distant peers. This isn't even just about the person, it also has a lot to do with how their life is currently going. My job requires me to constantly be learning new knowledge, and since the field I'm in is constantly changing and nobody's quite sure yet how to organize it, we need to be constantly adapting otherwise we risk not being of use anymore. There is a certain risk: if you're not quick to think or act, or if you can't handle organizing a mess, it doesn't really end well. It definitely isn't for everyone.
Meanwhile, some jobs require you do follow a specific path and leave no room for change whatsoever. They lean a lot more heavily towards tradition and "whatever has worked in the past". Being in such an environment for too long (specially if you have to deal with incredibly annoying managers/bosses and stuff) can make your brain start to think more restrictively and literally stop considering or even thinking about alternative solutions. I guess that's called "Learned helplessness", you'd be so used to that way of thinking that you wouldn't even try something else until someone shows you that yes, there is an alternative. Though again, this definitely varies from person to person.
It's also got a bit to do with cultures in general. Many western cultures value confidence and assertion a lot, which requires having at least some form of resistance to change and "hardheadedness" if that makes any sense. If you're the type of person to have strong opinions about things, then it's possible that you likely don't want those opinions to change so easily, or at least not without an argument. Meanwhile, a lot of eastern cultures value wisdom, social harmony and careful thinking/planning more (sometimes to an extreme degree), which means you need to be more open to change and more willing to listen rather than just to say whatever comes to mind.
My point is not to argue that anyone or any way of living is superior to anyone else, my point is simply that's there's many ways to live, and if the people who are around you seem to criticize your way of living for no good reason other than "it's different than theirs", then you need to be around different people. If you want to keep an open mind, then do it, there's nothing wrong about that.
As far as attitudes toward the body goes:
I still have a bit of the "Your body is a temple" mentality in me, but that can generate a bit of an internal conflict at times.
On one hand, the possibility of being anything you can imagine is one that the internet presents to us everyday, and it goes much further than just the imagining things by yourself. It's sort of an outlet to our incredibly free minds. Your sense of identity could be tied to a particular image/persona more strongly than your own. Wouldn't it be great to be whatever you wanted to be and not be restrained by the physical world? With things like DNA editing getting more and more research, a future where we could just choose traits we want seems to be getting closer and closer. Even if it's probably too far away for it to matter to any of us, that temptation is still present for many people in today's day an age.
But on the other hand, having these limitations add a sort of charm that is difficult to describe with words. Even with our limited capabilities, even with death approaching us all the same when we get older, we are still able to accomplish great things with our lives. Maybe, because we spend less time thinking about trivial things that in theory can't be changed, we focus on things that actually matter more in the long run. It's great to see what people can do with their lifes, not because they of the way they are, but despite that. It shows us that many aspects or ourselves are really just random and shouldn't matter as far as defining you as a person, specially compared to things you actually had to put effort towards. It makes your accomplishments that much more meaningful.
Would the solution be to let everyone be free to have any appearance they wanted to? Or would the solution be for us not have to worry about such things in the first place and focus on more important matters? I don't really know.
I could elaborate more on some of the points I made on this post but...I think I already wrote too much (how the hell did this become a 7+ paragraph text???).
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"The quieter you become, the more you are able to hear." --Ram Dass