A bit confused Re U.S school system.

I am currently reading a most enjoyable series where the protagonist is in seventh grade.

As a Brit I have not grasped the age of this pupil (student?)

In Britain we start school at the age of five and attend "primary one. WE stay in primary school up to primary seven when at the age of twelve we move on to secondary school. (The big school) This is where we proceed through secondary one to secondary four where we leave at sixteen. There is an option to stay on for an extra two years i.e until the age of eighteen but it is not mandatory.

I get confused when grade, junior,freshman, sophomore are mentioned as I cannot determine the age of the characters.

As the story am reading involves sport activity and regulation "sports bra" in the dress code it makes me wonder how old a seventh grader is. I had assumed they would not have hit puberty unless it was the equivalent of our primary seven (twelve)

Something is lost in translation for this ignorant Brit. Could someone give me a rough guide of the grades and age progression through the U.S. school system?

I would be most grateful. Kindest regards WebDeb.