I agree with Thorax on all counts.
People of older generations look down on your (and I suppose my generation) because of the way we behave. [From hereon, understand that when I say "we," I mean the majority of individuals aged 13-24, who all supposedly fall into generation Z]. Due to the fact that as children we had most comforts in life handed to us by our parents (thanklessly at that) and the fact that we belong to the "everyone is special/a winner" upbringing, we act as though everything should be provided for us. We are extremely self-entitled and self-centric. Our generation has not yet had to grasp with societal or historical shifts that events like WWII, Vietnam, or Cold War tension inflicted on our elders. These events served to develop them characteristically and make them, for the most part, a more steadfast group.
Let's face it. We are spoiled. We are demanding. And in your case, we expect respect to be given to us without having to earn it. If someone disagrees with us, rather than seeing it as an opportunity to understand a different perspective, we consider it an attack and react with bratty behavior and rebellion. You clearly showed this in your reaction to Thorax's comment where you called him "loving dense" and claimed he didn't make any sense. Who are you to determine that? You are 15. You have not experienced very much yet. You do not have a vast collection of experiences, traumas, successes, failures, and character building events to reference. Yet you insist that you are more intelligent and more deserving of fair treatment than your elders.
It is a very arrogant viewpoint. Understand that there is always another perspective contrary to yours which is valid. Also understand that it is not just you and your generation that are being looked down upon. Human behavior is constant. Every generation has gone through a period of self-important behavior and been knocked down a couple pegs to realize how much of a little stuff they were being. Eventually you will grow to hold distaste for your subordinate generation and be thankful that you no longer behave in such a manner.
TL;DR - Old people look at you this way because you act like a spoiled brat. Eventually you will grow out of it.