You will need a degree in fashion design from college in order to become a fashion designer. An associate or Bachelor's degree will do. You will also need training in business. A designer will need a skill for textures and design as well.
Any one of any age can be a fashion designer. However just because you designed a fashion does not mean anyone will wear it or pay for it. For that you need to become a great fashion designer. For that you will need practice, probably some schooling in fashion. and a lot of luck.