Accept that you can be wrong, and that there's nothing wrong with that, because everyone is sometimes. That's your first step to really growing up. You'll be mature when you stop trying to show and prove how mature you are. What you're doing now just makes you look ignorant and childish.
Take a second and think, plan and review in your head before you type, or speak for that matter. It'll do you a world of good.
These are amongst the truest things that I've ever seen on these forums.
And kingcobradude: seriously, just listen to some of these things. For example, what maddafinga posted is just SUCH good advice that you'd be missing out on loads not to listen to it.
As I look back at how I've changed over the past few years (I'm only 18), a sizeable chunk of what I see is summed up in this one sentence by maddafinga:
"You'll be mature when you stop trying to show and prove how mature you are."