10-12 years old.  Kids start kindergarten around age 5, proceed from grades 1 through 8, then 4 years of high school, then 4 or more years of college to a baccalaureate degree, or on to postgraduate work.  Some American school districts now seem to divide grades 1-8 between "elementary school" (K-4) and "middle school" (grades 5-8), but that wasn't my experience.

Sergio Riolo wrote:

Reading about young people visiting American museums, I've come through
"5th or 6th grade students". Can anyone tell me how old is people
attending these school grades?
Many thanks,

Sergio Riolo
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