Teenagers act rude and dumb because they are at a stage were they are trying to discover and explore many things to help determine who they really are. It's also a phase were they feel they are no longer kids that need to be control by parents, so they explore things that they couldn't when they were a lot younger.
As stated in Eric Erickson's psychosocial theory, at teenage stage (ages 12–18), teenagers are faced with the task of identity vs. role confusion. Teenagers struggle with questions such as “Who am I?” and “What do I want to do with my life?” most of them take on different selves to see which ones fits best. They try to discover and explore several roles, ideas, set goals.
Teenagers are often rude and dumb because they are trying to discover who they are apart from their parents. Teenagers will often demonstrate this separation by challenging the rules and arguing with their parents. Sometimes teenage backtalk merely is them trying to figure out who they are.
Also, the teenager will begin to lose their childhood perspective that their parents know everything. Most of the time, the parent will start to see that their opinion that once meant the world to their child, will begin to matter less and less as they will often seek approval from their peers.