Prospective clients may prefer to see some evidence of training, even to the point of certification. It may not be enough just to tell them you’re good at it and you love it, or even to show samples (which might or might not be your own work). If you’re serious, a credential is always a good thing to have.
An instructional environment also offers you exposure to the breadth of the field and helps you wend your way among alternatives. Self-teaching is good, but to develop your understanding in a systematic fashion, to receive guidance in what is mainstream and respected, what is fringe thinking, what is passe, etc., and to get useful correction, you need a teacher. This is why for all my study in linguistics and all the books I have read, I do not have an orderly grasp of the field.
I presume you’re working. Where I live there are many fine community colleges, university extension courses, and other institutions that offer evening and weekend programs toward credentials in technical and other subjects. There is also distance learning. You might look into schools in your area and see what options they offer for an adult population. One or two classes a semester should not be too expensive at such places, and plenty of people have worked full time and carried a busy course schedule to get through school..
You are very young and have plenty of time, so don’t worry. My mother got her master’s at 51 after raising a family and went on to teach at a renowned university. My aunt went back to school at 61 for her Ph.D. in linguistics. I think it is never too late.
And I doubt that you will ever be sorry for having pursued science, even on the rebound. It will give you a special slant on graphic design that few practitioners have. It may also enhance your qualifications to work in graphics in a high-tech environment, if that interests you.