Is it illegal to buy lighter fluid if you are under 18 in California?

Most likely. The law and "children" is designed to keep hazardous materials out of the hands of children. After 18, it is presumed that you will not do stupid stuff like try to blow up the outhouse with lighter fluid. It also presumes that it will prevent smoking in an indirect way. If you have a refillable lighter and you can't refill it, what use is it? Lighter fluid is a hazardous material because of its low flash point, so the law is most likely tied to the federal ones about hazardous materials.