Funny to read someone else's musings on this as I've been working through many of the same questions for a while. I started full-time in the tech industry before I was out of high school, then went corporate, then got out for a while, then back in, then had my own business for a while, sold that, got a job for a few years, and recently left that job and went traveling for a bit to think about things and consider going back to full-time employment or work on a project and hope it makes enough to survive on.
> Do I have to choose it now, or can I do what I’m doing now and decide later?
If you're thinking about it now, then you should start figuring it out, or you're postponing the inevitable and these questions will be on your mind every day.
> If so, why are these questions disturbing me now and not in the future?
Possibly because you are dissatisfied in some way with what you're doing now. A good chance that you're finding it comfortable but not stimulating.
> How do most people work out what’s the right path for them to take?
Most people don't, and don't believe the ones that say they knew in advance what the right decisions were. None of us can see clearly into our possible futures; we can't see in advance the successes we'll have or the mistakes we'll make. The best we can do is to understand our motivations when we do make a decision, and then at least be at peace with that decision no matter how it turns out.
> Does it have to be either or can you do both things together—have a full-time job and do your own thing?
If you want to live a balanced life -- i.e., without all of your waking hours behind computer screens -- you should choose either/or. Working full-time and trying to put real energy into a side project doesn't leave much time for anything else, and it will cause both your full-time work and your side project to suffer.
This isn't to say that people never manage to pull this off. Some people make it work out. They are the exceptions that prove the rule.
> Looking back to the twenty-year-old me and the person I’ve now become, I see almost no similarities, interests, or passion. I was a completely different person back then and the only thing that has stuck with me after all these years is my love of soccer. How do people predict the future? Clearly, I’m no good at it.
Congratulations, you're a normal, healthy person that's still growing and maturing. Keep doing that.
> I wonder whether, as we grow older, the rate of change in our interests slows down too?
It hasn't, for me, and I think I'm at least 10 years ahead of you. Nor has it for any of my more interesting older friends.
What does change is the gradual realization that you won't have time for all of your interests. Being a dilletante is a lot of fun while in your 20s and 30s. Some manage to have fun continuing to dabble in a bit of everything for many more years, even into very old age. Other folks start to feel like they might be missing out on something by just grazing the surface of their interests, and start to consider whether their time might be better spent focusing more intently on just a few.
> I imagine working on your own thing full time and on the side are definitely not the same; it’s scary, to be frank. If my side-project became my full-time work, might my passion for my project dwindle and become yet another job? How would it differ from my current job?
Your project should become your full time job. You should maintain the discipline you have in your current job and resist the urge to fritter time away here and there.
But it will be different, because you'll be investing in your interest, and despite it becoming your job, you'll still feel an ongoing sense of satisfaction at being solely responsible for your success or your failure. If you wake up one morning and have a great idea for your project, you won't have to convince middle or upper management, or enlist a sales team, or wait until the next company all-hands. You'll just do it.
And that can be really rewarding, and it can also be a great big trap you fall into, without anybody else holding you accountable or challenging whether your next idea is a good one or not.