There is a little bit of scientific truth behind the association of maturity with age; the prefrontal cortex is not fully developed until a person is 25. That being said, there is much diversity among individuals and it is possible for a teenager to be more mature, intelligent, and knowledgeable than a person older than 25.
Cognitive function and the ability to reason and conceptualize occur earlier in some children than they do in others. As early as 2 years old, the human brain has the capacity to exhibit the early signs of empathy, and the ability to understand feelings from the perspective of others appears as early as 3. The centers of the brain controlling physical coordination, emotion, and motivation are all fully developed by the time a person reaches their late teens. Interestingly enough, the last center of the brain to fully develop is that which controls judgement--aka the prefrontal cortex, as mentioned before. Given the facts of brain development science provides us with, I believe it is totally demeaning to "talk down" to younger adolescents. A lot of adults who do this are really letting their own insecurity get the best of them; using the fact that they are older as a display of superiority to boost their own ego. People who behave like this should really question their own maturity. I believe a fourteen year old should not be denied the chance of exercising his/her capacity to reason any more than a 20 or 30 year old. Why are we really struggling over the question of maturity in the first place? Perhaps it is more accurate to say that maturity doesn't just happen with age, but it "develops over time" *with patience, engagement, and actively exercising the brain centers associated with it.*
Citations:
http://www.npr.org/templates/story/story.php?storyId=141164708
http://teenbrain.drugfree.org/science/behavior.html
http://www.scholastic.com/teachers/article/ages-stages-empathy