> Here in Spain, everyone born in the same year goes to the same class, is it different in USA?

So Spain has a cut date of 1 Jan while the USA is 1 Sept. The overall effect is exactly the same: When a school term begins, some of the students will be nearly 1 year older than some of the others.

As per the article itself, many states implement the "1st of September cut date". A kid born in September has to wait an entire year to start school compared to one born on 08/31.