Why do people instantly assume that teenagers don't understand reality?

Not all teenagers are immature! Some have been through a great deal in their life meaning they have a lot of experience. But why is it most adults look down on teens?

Examples are welcome...
Update: I am not saying there is no need for teenagers to respect adults because of course there is without respect the world would be in turmoil!

Remember not all teenagers play video games and watch stupidly unreal TV programs. I know I never have and I know many of my mates are against them.
Update 2: Reality is not a paradox it is our life!

And teenagers do not all disrespect adults and if they do 98% of those who have no respect were brought up by morons who have no respect for others themselves!
Update 3: By experience I can use myself as an example: 1. Suffer from Manic Depression aka Bipolar since I was 11. 2. Have been sexually assaulted! 3. Have had such a low self-esteem in the past that turned to drugs and drinking because I wanted to die! 4. Had miscarriage at 14 from a pregnancy that was the... show more By experience I can use myself as an example:

1. Suffer from Manic Depression aka Bipolar since I was 11.

2. Have been sexually assaulted!

3. Have had such a low self-esteem in the past that turned to drugs and drinking because I wanted to die!

4. Had miscarriage at 14 from a pregnancy that was the result of being sexually assaulted.

5. Made a big step forwards by admitting how bad my depression is and am on the rocky path of trying to find the right thing to help me.

And many other things have happened...

And I know I actually have a good life compared to other people in the world but what I have and still am having to go through is more than some adults have.

NO I AM NOT PRIORITISING IN ANY WAY!
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