There are various types of student visas depending on the age of the student and how long they intend to stay and study for in the United Kingdom for example child students can apply for and come to the United Kingdom to study between the ages of 4 and 17 however this must be a an independent fee paying school. One of the more popular student visa categories is the Tier 4 (General) Student Migrant category which is not restricted to a particular age or particular length of student.
Also depending on the level of entry into the county will depend on whether you can switch from one student category to another. Other
student visas enable the holder to come to the UK as a prospective student to decide upon which course of study or if they plan to start a course of study within 6 months. For example if you come to the United Kingdom as a student visitor for up to six months you are not entitled to work and on completion of your studies if you wish to switch to the Tier 4 (General) student category so specific restrictions to holders of certain student visas.