I'm sure rules vary from state to state and even college to college. But you might want to look into co-enrollment (high school & college at the same time).
Also, you might see if you can audit the course. Which means you're allowed to take the course but you don't receive any "official" credit and you must pay like it's a regular class. It still wouldn't get you certified but it might make those 3 years of bed-making a little easier to take. It may also give you some good contacts and an idea of what areas are important to study so that you'll be able to focus on getting the ones you're allowed to take while you wait those 3 years, so that by the time you turn 18, you're already well on your way in that field of study. You might even be able to clep the course when you're 18, which means you take a final exam and if you pass it, you receive credit for the course without having to actually sit through 3-6 months of classroom instruction again; though I would imagine you would wish to just for a refresher, but you'd still be ahead of the rest of the class because you'd taken it before.
I've worked for a University for 8 years and I've seen colleges break their rules before. It's possible if you write to the Dean, requesting to take the course and you make a good case, you will be allowed to audit it. (And I do mean WRITE, Deans don't think much of phone calls).
I imagine the "18" rule may have something to do with the laws of your particular state, most especially the age at which you're considered a "legal" adult and the fact that you have to be one before you can sign any sort of contract. I haven't had any EMS experience, but I'm assuming some sort of contract must be signed, if only for insurance purposes, even by volunteers. Teenagers have a nasty habit of thinking they're indestructible (not to say YOU fall into that category so it's just as much protection for them as for you.
Just a few thoughts from someone on the college-end, rather than EMT end of things.
so it's just as much protection for them as for you.