Full-time Education is not limited to Primary and Secondary Education only; it also encompasses students in Further and Higher Education i.e. colleges and universities. The phrase does not indicate the age of the student. An 80 year-old can enrol in full-time education.
Generally "full-time" is a measure of the amount of time a course takes up and is commonly defined thus:
Further Education - enrolled on programmes of at least 450 guided learning hours per year, or for at least 150 guided learning hours per tri-annual period or more than 16 guided learning hours per week for shorter courses.
Higher Education - enrolled on programmes of study involving a minimum of 24 weeks per year.
Unfortunately this ambiguous term should never be used to signify when maintenance support will end without reference to a specific maximum age.