Very good answers thus far. Just a few additional thoughts and comments on some aspects of your questions.
No, you don't have to go to college. However, I'd strongly recommend it. And, really, not so much in order to reach your goal of becoming a Realtor, though it won't hurt. But if you change your career focus, it may become extremely important to have a degree. And even if you stay with real estate, there are many benefits to college beyond just the absolute knowledge you'll pick up. Look: Do both: Go to college AND become a Realtor. People do that, too.
You ask about practice exams. Yes, there are books full of practice exams and practice questions. There are even computerized programs. I don't know how it is in your area, but in Virginia the exams are on computer. The practice exams simulate that experience exactly. Further, the computer practice programs are neat because you can specify a greater emphasis one one area or another (an area you're weak in) of real estate. And the practice exams will show you your strengths and weaknesses. Also, when I went through the process (with a real estate firm), the firm required that all prospective agents take and pass a practice exam from the firm before taking the real thing. That was part of the 60 hours of state-mandated education we were required to take. So: Don't worry about practice questions and exams. There's plenty of that.
As for whether someone will take you seriously. It'll be more difficult than if you were, let's say, 40 years old. But not impossible at all. I've known some very successful agents in their 20s. I'm also active as a real estate investor, and I know plenty of investors who started off in their late teens. In fact, that's what my son--now 19--is doing. On the one hand, it's true that people can make quick decisions about someone's competence based, in part, on the person's appearance. On the other hand, what buyers and sellers ultimately are looking for is someone who knows what he/she is doing. And, from a sales perspective, they're looking for someone they can trust and have confidence in. If you know your stuff and come across as a true professional, age is not a major barrier.