What's with parents always thinking they are always right?

Why do parents always think that they are always right just because they have a bit more experience than their kids? I'm not trying to act like a rotten child or anything, but surely parents can learn a thing or two from their own children. They always use the "experience" excuse, but I find that excuse obsolete because what they experienced back in the days are not exactly the same as it is today. I am 18 and I am tired of my parents not taking me seriously. And for people that says that I am too still too young and is still immature, then can you explain immature adults( people that are over 25)? You cannot deny that adults do not act immaturely, because I have seen quite a few in my 18 years of life. Now if there is such thing as immature adults, then why not the idea of mature children?