Writing is free. Anyone who tries to tell you differently is selling something.
But in all seriousness, I've seen enough backhanded compliments to writers and read enough angry posts on forums to know that there are some people who believe in "dues." What are dues exactly? From what I've gleaned they are: being old enough, being rejected enough, writing long enough, writing well enough, being humble enough, being rockstar enough, being involved enough, or all of the above.
But I hate to break it to you, dues are a figment of your imagination. This writing gig? Anyone can do it. Now I'm not saying everyone who writes a book can or should get published, and there are a lot of fabulous writers out there who only concern themselves with writing. But if you are holding yourself back from writing because you're young or you don't have a degree or you're pretty sure people will laugh at you, you're doing yourself a disservice.
There will always be naysayers. Those people that scoff at the teenager who is writing a novel. Those that will post nasty messages about writers who get book deals with seemingly no effort (no one gets a book deal without effort -- even celebrities have to find ghost writers). Others who will say you aren't a real writer until you've been rejected dozens of times for representation or publication.
And then they'll say something about dues. About how they've paid them and you haven't.
Here's the thing though: You don't have to pay dues to write a book or land an agent or get a book deal.
Your dedication - your persistence - your passion: that's all that's required. And you don't owe it to anyone but yourself.