I prefer the junior high/high school system, but that could be just because that’s what I had in Virginia in the 1980s.
As for ages, they can start 1st grade at age 6, which means a student whose birthday is in the summer (like mine) completes the final year of secondary school at age 17 and turns 18 before starting college in the fall.
Unless they're your kids, in which case they're probably doing college work at fourteen.