Sometimes You Just Don't Know Until You Start
Your career is much the same. In all the years I've worked with people on career direction, I've only met one who knew from a very young age what he wanted to be. The rest of the people I've worked with have had to look for their path, sometimes finding that one path isn't exactly what they were looking for then moving toward another.
Starting a business is much the same. You can write it all down ahead of time, and estimate your costs and your income, but you won't know that your business will be viable until it's viable. You just start and give it your best shot, and see what happens. As you actually carry out your idea, you can see what works and what doesn't. You'll find out what's possible as it actually happens.
I have several clients who believe that they should know what they want to do, and they heap a lot of guilt and doubt upon themselves for not knowing. My encouragement to them is to go ahead and explore an option and see where it goes. You have the freedom to try things, learn from them, and build a body of experience that will grow in value and opportunity as you go. You have the freedom to follow a whim and see where it leads, even if it seems ludicrous. And you have the freedom to look until you're satisfied with where you land.
It's impossible to know exactly how things are going to turn out ahead of time. You won't know until you go ahead and do it. So go ahead and do it now.