There has to be a line where you can classify somebody as an adult. While teenagers might believe they are mature, in my experience that is just what it is, something they believe and not something that is true. Almost everybody I knew growing up and almost everybody that hasn't reached 18 years old are with very few exceptions dumb kids that don't have the common sense or life experience they need to make adult decisions. I spent a lot of my time just observing them around me. The vast majority of them cared more about where the next party was, who they could get to buy alcohol for them, or which chick they could screw rather than any serious matter in their life. That was the biggest reason I never socialized with them, they acted like stupid kids. Oh sure almost every single one of them would have said that they thought they were all grown up, but most of them now that they really have become an adult would probably admit they did some pretty stupid stuff they wouldn't have done if they knew then what they know now. They recognize what they thought and did back then wasn't because of maturity but because they were just being stupid. Just because teens do things doesn't mean they should.
What's ironic was that the few people that acted older than they were at that age didn't have any problems waiting a few more years and realized just how inexperienced they were in life. Most of them didn't want to do what everybody else did or weren't concerned yet with all the other people thought they were ready for. It's an extremely rare 14 year old that is anywhere near that mature to handle those sorts of things, especially here. I can maybe see in a war zone or a third world country where people might end up growing up faster than they otherwise would, but with the lives we lead.
Centuries ago I could give people a break for marrying young and other things because people of that era didn’t live that long and by necessity had to do things like start a family in their early teens. For much of the world that isn't the case anymore.
I wonder, if people that really believe people that young are adults and capable of making any decision they wanted, would you be willing to let them have sex with any older person they wanted? Would you let them vote in important elections? Would you let them buy all the alcohol they wanted, let them smoke, and let them buy porn? Would you believe they were old enough to make a decision about joining the military so they could go die in some God forsaken country half way around the world? Would you think children of that age are mature enough to go in and buy a firearm whenever they wanted? Could they get a driver’s license and go drive a car whenever they wanted?
It might be unfair to the very very very few people that are young and have the maturity of an adult but the fact is that the vast majority of them are still kids and that's why in most places they are treated as such. It isn't possible for society to go around and somehow determine how capable each and every person is at a young age. That is why you need to have an age of adulthood where you get the rights of an adult, and in my opinion that sure isn't 14, 15, or even 17 years old.