A gap year is just a fancy term for not studying for an academic year. You can choose to do this at any time. However the older you get, the more life's pressures demand your time and attention, which is why most people who have the chance go straight through from school to college / university without any break, then by the time you hit 18 or 21 you have a good array of qualifications that will get you where you want to be - then you can take as many gap years as you want, Most people opt for a gap two weeks every year ie a good holiday.