How Old Is The Average 9Th Grader. 5/5 ( 21 votes ) twelfth grade, 12th grade, senior year, or grade 12 is the final year of secondary school in most of north america. E.g., 9th grade + 5/6years = 14/15 years old.
In addition to being basically the same weight, 9 year old boys and girls are also basically the same height. How old is the average ninth grader? Although a child may be growing, his/her growth pattern may deviate from the normal.