I agree that marriage has nothing to do with age.
I'm twenty years old and have been with my current boyfriend for four years. Neither of us has ever cheated, we've had fights as all couples do but we managed to work through them. He is more than my best friend. He's been with me through a lot of really hard times. I have social anxiety disorder and despite all my problems he has stuck by my side and we are more in love now than we ever were.
Just because we're twenty does not mean anything. If we were thirty and had only known each other a year would you think we were more prepared to get married? People change throughout their entire lives, I watched my mother change significantly after my dad died and she was nearly fifty. We are both incredibly mature for our age. We spend enough time together that we know everything bad and good about each other.
And if you want to throw statistics in there, most couples that live together before they're married get divorced.
We're both faithful so the idea of sleeping around and "playing the feild" isn't for me. We're not engaged yet but we've pretty much decided we're going to get married. We both want kids. We are both very family oriented and want to stay where we grew up. We love each others families (in fact he will often times babysit my nephews when I can't). We're both in college right now but both of our dreams are artistic (I want to go into film and he wants to be a musician we'll both probably end up as teachers) so if we wait for financial stability we'll probably be waiting our whole lives.
I understand that at 20 years old many people aren't ready, but to group everyone together by age is unfair.