A freshman is 9th grade - 14 - 15 yrs. old. A junior is in the 11th year of school./ A senior is in the 12th and final year of high school. Ages vary but a junior is 16-17 years old and a senior is 17-18 years old. Junior is 11 grade, senior is 12 grade (for high school) A junior would be around 17 and a senior around 18.