How can people ... adult, not in their early 20s etc ... afford to quit their job
Depending on what you did the first 20 years of your career, it isn't unreasonable to be 45 and have enough savings that going without pay for 12 months is feasible, especially if you come from a well off background. This is doubly true if you have a partner that also has a job. Also at 45 you hopefully have enough professional contacts that getting a new job if your business fails isn't impossible.
Another option is to bootstrap in your free time and then quit your job once you have your first customer. I've had at least a couple of colleagues that have done that.
I could maybe do 12 months without bringing in a paycheque by digging into savings, for sure.
The problem is that to properly bootstrap does mean there's things you'll need to pay out of pocket for that aren't just your own labour: for me this would be things like UX / graphic design, hosting costs, etc. That stuff can add up.
Depending on what you did the first 20 years of your career, it isn't unreasonable to be 45 and have enough savings that going without pay for 12 months is feasible, especially if you come from a well off background. This is doubly true if you have a partner that also has a job. Also at 45 you hopefully have enough professional contacts that getting a new job if your business fails isn't impossible.
Another option is to bootstrap in your free time and then quit your job once you have your first customer. I've had at least a couple of colleagues that have done that.