In the United States, students typically start high school at 9th grade, also known as freshman year, when they are about 14 or 15 years old. This age can vary slightly depending on the student's birthday and other factors such as early or delayed school enrollment. High school generally includes students from ages 14 to 18, covering grades 9 through 12. Freshman year students are usually around 14 years old at the beginning of the school year, transitioning from middle school to high school. This age range allows for some variation but 14-15 years old is the common starting age for high school in the US.