I’m not in college, but I’ve had three older siblings in college/university, so I hope this helps 
1. How old are the students studying in schools, colleges , universities?
It depends on the person and whether they want to go to college right away, or wait a year or two (or not go at all). All of my siblings went straight to college after they graduated high school when they were 18/19 years old. On the other hand, when I graduate high school, I’m planning on taking a “gap” year before going to college, which is just taking one year off from school to do things like make money, or look into more colleges and what you would like to do in future years.
2. What are the subjects in the universities and the students choose them and on what basis?
College students can major or minor in subjects. For example, my older brother majors in computer science (I don’t remember what his minor
), and one of my older sister majors in psychology, and minors in music. I believe they take other classes as well, like history, math, etc., but I don’t remember that well 
3. How are different semesters held , what is the gap between them…?
Throughout my school career, and it’s basically the same in college, there are 2 semesters, with 2 quarters in each. So one year is composed of 2 semesters, which is divided into 4 quarters, 2 quarters in 1 semester. Usually, the first semester ends with “Winter Break,” which is basically a break time for holidays and New Years, stuff like that, and then after we come back, the new semester begins. It can be different though, where after Winter Break ends, there may be 2-3 weeks before the new semester begins.
Hope this helps
