Some don't fully realize maturity or development until their mid-twenties. That being said, I can only speak for myself. And if I were balding at seventeen, I would only be using minoxidil. I would only consider using finasteride at 21-years of age, and beyond.
have u been growing for the past couple of years? i havent grown since i was 15 so taking finasteride at 17 probably wouldnt be a problem for me! if you've looked the same for the past year or two, try 0.25mg of finasteride then increase it up to 1mg a day once you get older. otherwise, wait until you've matured to start finasteride.