The short answer is anybody. There are no rules regarding who can become a freelancer. As long as you have an ability or skill that a company or private person needs for a particular service in a given time is called freelancing skill or job.
Freelancers are hired by individuals and organizations, including micro enterprises, little as well as moderate sized ventures, huge companies, like Tesco, charities, regional as well as central government, e.g., HM Income & Customs, and also public bodies.