Since I thought the answer to this might be useful, I screen capped your long ask so I could post it publicly! Made you anon as requested. =)
In the US, you do not need to register in any fashion to begin a career as a freelance illustrator. But you do need to pay taxes on every job you do, even the small ones. Each state has different tax laws, and there are of course the federal tax laws to consider. So it’s important that each year when you file your taxes as a freelancer, you abide by those laws. We files taxes once per year, or pay into our taxes during a quarterly statement (every three months) to avoid paying a large amount at the end of the tax year.
As for guaranteed retirement, well… there really is no guaranteed retirement in life. All kinds of things can get in the way of retiring, from medical expenses, to identify theft, to loosing the job that’s supposed to pay for that retirement. As a freelancer, you are responsible for every single aspect of your financial planning, including retirement. If you plan properly, don’t face disaster, and keep up the good work, perhaps retirement is in your future as a freelancer! But maybe also you don’t make enough, and you need to change careers, or have two jobs. Freelancer illustration isn’t exactly known for it’s stability as a career, but it IS possible to make a decent living off of it and perhaps retire. Here’s hoping, right? ;)
I recommend this book to future freelancers, it helped me organize my finances a lot! Thanks to Thornwolf for getting this book for me when I was at a loss on how to manage the financial side of my business. It’s helped me a lot.