Is 17 the last year of high school and if it is than why?

Question

Update:

I’m talking about USA schools 

Answer 1

In US, 18 is  usually last year of HS. Some graduate early ; or drop out ; or join service at 16.

Answer 2

A student who is a senior in high school is usually 17 or 18 years old. 

Why? Because students start at age 5/6 and once they are done with all grades Kindergarten- 12th grade they have reached 17/18 years old.

Answer 3

the relationship between age and graduation is not necessarily fixed

Answer 4

year 4 was my last year