After 8th grade is 9th grade which is also called freshman year because it is the first year of high school in the United States. High school consists of grades 9 to 12 and is what students that complete 8th grade move on to after completing middle school/junior high.
Students in eighth grade are 13 or 14 years of age.
Students usually begin eighth grade at 13 years old and turn 14 by the end of the school year.
If you are wondering about high school then I suggest you read this article (table included) I recently wrote.